Names of Notes – These are specific names to each sound emitted from the instrument, each of which is unique.
Rhythm – This is the organization of sounds into clusters of beats.
Key Signature – This describes the organization of pitches to fit into sharps and flats.
Time Signature – This establishes the organization of notes into a number of counts for each group.
In addition to this, music theory also teaches you about the following
Chord structure and chord progression
The major and minor keys and their differences
Complex chord and rhythmic pattern ...