Music theory begins with the very basics of reading music - notes on the staff, key and basic common time signatures. Turning the little dots on the page into sounds. But it is so much more!!!  It gets into more complex concepts such as the 6/8, 3/8 time signatures, Circle of Fifths, intervals, chords, the less used scales (the various minor scales, blues scales), various common more complex rhythms, rests, accidentals, dynamics, tempo, legato/staccato, slurs, phrases, song endings, chord progressions... and so much more!!!
 
What?!  OK - what IS all that other stuff?  
It's all the nuts and bolts of music.  It's what makes a player of an instrument into a musician!!  It allows them to understand the patterns found in all music, to fully understand the complexity and nuances of what they are playing, to better see how their part fits into the larger (orchestra/band), to better articulate the art of music and to better grasp music as basically a whole different language.
 
While our ensemble classes (band, orchestra, chorus and guitar) cover by necessity the basics, there is little chance in those arenas to get into the more detailed and complex aspects of music theory. $210