Q: I've taken theory III and I still don't understand tritone substitution can you explain it to me
A: imagine you are in a song, minding your own business, ... let's say the song is in the key of C ... and let's say there is  a G7 chord.  .. following me so far? the G7 chord has notes b and f in the mid-range, that's the important notes of the chord. (that's a tritone interval but that's not why it's called substitution, we haven't done the substitution yet, that will be in the bass.) so let's say you play b and f notes (at the same time) in the middle range, and the bass plays G (note.) that pretty much makes a G7 chord. ok now the bass player plays a prank and plays Db (note) instead, while the piano still plays the b+f notes. pow, now it's a Db7 chord