Okay, I'll try to help!

My background: At about 14, one night with the radio on, on the way to sleep, I heard some music and thought "WOW!". It was Beethoven's 7th Symphony. It can be a lot of fun -- at least one part is great dance music.

Then I kept listening, buying records, seeing what I liked and didn't. I found a lot to like.

As a math grad student at Indiana University, my dorm was next to the excellent music school they had (have?) there with a lot of orchestra and soloist quality music students. One, a Stern protege, put his old Italian violin under my left chin and gave me a first lesson. Soon I took a "course" in violin, got a loan of a not very valuable violin from the school, started at the beginning tuning the violin. The teaching was a good start; my teacher was terrific, played the Brahms concerto in Toronto!! I continued and eventually made it through parts of my favorite music. I wasn't any good, but as far as I got was fun beyond belief -- could scream out to the heavens with the full passion of the human spirit!! The violin gave me a much better voice for such screaming out!

So, for Bach, right, for the
prelude to the third partita for
solo violin, e.g.,

https://www.youtube.com/watch?v=2KYRdRnnBYw

So, it's super famous and a favorite of violinists. It starts with a haralding call, rushes through lots of running around gymnastics, and ends with a nice calm resolution.

I did well enough on the first two pages and the fourth one -- still need work on the third page! And I got through about half of the Chaconne.

So, let's start with a standard favorite, easy enough to like that it is close "pop" music:

https://www.youtube.com/watch?v=jf_ADv_Fnlo

So, right, this is the prelude to one of the Bach pieces for solo cello. There's an obvious way to play it on violin, and I got through that.

So, what is going on here? Well, opinions will vary, but here's my view: The piece is about some guy thinking about something. He cares; he's passionate. Maybe he is thinking about some problem in his relationship with his girlfriend. He starts out confused. He things of X, interrupts thinking of Y, keeps charging on considering this and that, with some thoughts interrupting others, some thoughts with some momentum of their own. Finally he begins to understand and get to a solution. Then he gets really happy! He rises with victory, joy! It's like he just won the 100 meters at the Olympics, takes a victory lap with his arms above his head, and sees his girl in the stands really happy with him and cheering!!!

Or, the piece is a short drama as in formula fiction: He's a nice guy, the protagonist, has a problem he keeps struggling with, and finally gets a great solution and wins the girl.

Imagine that the guy is explaining all this to you, from the problem though his struggles, to his solution and victory. He's passionate, really wants a solution. BUT!!!!! He is explaining this to you in a language you don't know even one word of! So, you can't understand his words, but you can hear his passion. To me, that is what Bach is doing in that music.

And to me that is mostly what Bach does: So, there is some speech in a natural language, maybe English, French, Russian, whatever, fervent, emotional, maybe passionate, speech, and, since we don't understand the language or the words, all we get is just the emotion.

The speech can be, as from that cello piece, by just one person. Or the speech can be several people, an orchestra or choir, all saying much the same thing. Or the speech can be a dialog between two persons or groups of persons.

Sure, my favorite piece of music is the Bach Chaconne. So, it's the last part, sort of tacked on, to one of his six solo pieces for violin. It's written in the keys of D minor and D major.

Keys? Okay, take a sound wave that repeats, say, 440 times a second. It will have a pitch like the A above middle C on a piano. You can buy a tuning fork for 440 Hz A -- it's a standard. Well, 880 Hz is also an A but an octave higher. Okay, 880 / 440 = 2, so in going up an octave we have gone up a factor of 2 in frequency. Well, we can consider 2^(1/12), that is, the 12th root of 2. So, multiply that by itself 12 times and will get 2. So, take A at 440 and multiply the frequency by this 2^(1/12) thingy, and will get, A#, that is, one half step or one semi-tone up from A. Multiply again and get two half steps up or a full step or B. Or B is two half steps above A, two half tones, two semi-tones, one whole tone. So to go up a semi-tone, multiply the frequency by 2^(1/12). To go up a whole tone, multiply that by 2^(1/12) twice.

So, start on A at 440 Hz and go up tone, tone, semi-tone, tone, tone, tone, semi-tone, will have gone up 12 semi-tones, and will be at A at 880 Hz. You now have the notes of the A major scale, the notes of the key of A major! The notes are A, B, C#, D, E, F#, G#, A. Can do these steps starting with any note on the piano, say, C, and get the C major scale, the notes of the key of C major, and get the notes C, D, E, F, G, A, B, C, which are all the white keys on a piano.

Or, for D major, get D, E, F#, G, A, B, C#, D.

An interval of a fifth is seven semi-tones. So, if start with C and go up a fifth, get G. So the notes of G major are G, A, B, C, D, E, F#, G. So, we have one sharp! If we go up a fifth from G we get D, and as we have seen the notes of D major are D, E, F#, G, A, B, C#, D. So, we have two sharps, the one we had from G major plus one more. If we go up from D a fifth we get to A with notes, as we saw, A, B, C#, D, E, F#, G#, A, or three sharps, the two we had with D major plus one more, G#. We can continue this and get the "circle of fifth". Cute.

But if we start on D and go up tone, semi-tone, tone, tone, tone, tone, semi-tone, then again we have gone up 12 semi-tones and get notes D, E, F, G, A, B, C#, D, the D minor scale.

This use of 2^(1/12) to define the notes of, say, a piano keyboard is good, called tempered tuning, but not the only way. Here's the core of the main other way called perfect tuning: We can observe that

2^(7/12) = 1.49830707688... ~ 1.5 = 3/2

So, going up 7 semi-tones, a fifth, multiplies frequency by right at 1.5 = 3/2. Well, a violin has four strings. The one on the far left for the violinist (as the instrument is held) is the first G below middle C. The next string to the right is D, then A and E. So, each adjacent pair of strings is separated by a fifth and will have a frequency ratio right at 3/2. But with 3/2, three times the frequency of the string with the lower frequency has the same frequency as two times the frequency of the higher string. So, the second overtone (think Fourier series for periodic waves) of the lower string will have the same frequency as the first overtone of the higher frequency. Well, if bow the two strings at the same time, then can hear this common frequency or, if the two frequencies are not quite equal, the beats in the two frequencies. So a violinist tunes his instrument by adjusting to make the beats go away. He starts with the A string at 440 Hz then plays the A and E strings and adjusts the frequency of the E string. Then the same for the A and D and then for the D and G.

So, we now have four notes -- G, D, A, E -- tuned to have ratios of 3/2. So, we have a ratio of small whole numbers. Continuing in this way, we can work to get nearly all the notes on a violin by working with ratios of small whole numbers. Those ratios of small whole numbers are close to whole number powers of our 2^(1/12).

In the history of music, violins, flutes, trumpets were tuned with ratios of small whole numbers. But, if want to build a piano or organ that can play the major and minor scales with any key, then get a big mess -- need too many close but significantly different frequencies. So, if just use 2^(1/12) or approximately that, then can get "tempered" tuning as on current pianos, and each octave needs only 12 frequencies. Bach helped make tempered tuning accepted.

Simple recipe for music: Pick a note, say, D, and pick either D major or D minor. Play D. Then play all the other notes in that scale except D. Then return to D. As long as you don't return to D, the music will seem to have no end. As soon as you do return to D, the music will seem ended. First cut, the D major version will sound glad, and the D minor version, sad. Same for all the other major and minor keys. In the key of D, the note D is called the tonic and is roughly the beginning and end of a piece in D.

Okay, the Bach Chaconne starts in D minor, has a center section in D major, and continues in D minor to the end. Yes, it starts and ends on D.

My favorite part of it is at the end of the center D major section. Here are a few seconds starting at the end of that section and continuing in last D minor section:

https://www.youtube.com/watch?v=RATaKVXsjgQ

After the climax part, the music gets confused: An artistic interpretation is that the confusion is release or catharsis from the climax. Not all violinists try for such!

Here is a Heifetz performance of the Chaconne:

https://www.youtube.com/watch?v=kJvYYBGYW50

So, what is an interpretation? Well, that is made a little easier by the fact that a chaconne is dance rhythm. So, the piece keeps repeating this rhythm or close to it for the full 13 minutes or so. So an effect is that the music is insistent. Then, in the sense of speech, the music is like someone saying much the same thing over and over but in somewhat different ways, with different emotions, over and over. So, the guy is thinking about heavy, passionate stuff, and trying and trying to get to some resolution.

The whole first D minor section does this, over and over, versions of being insistent, using the chaconne dance rhythm.

Then at the start of the center D major section, at about 6:35 in the link above, the music becomes calm, "glad", happier, like the guy is finally getting some happy memories (no doubt about his girl!). But the calm feelings don't last and, instead, build and build, with some very insistent notes played three times. After these triplets, the music returns to some of the beginning and builds, with astounding use of just the four strings on a violin to the climax, at about 9:20, to the end of the D major section at about 9:45, the catharsis, if believe that, and the final D minor section.

There are also good versions for piano, one by Brahms for left hand alone, full orchestra, etc.