Answer:
It is basically the "math" of music. It explains why certain note sounds as they do
Generally speaking, "theory," whether it be science or the arts, is the study of that discipline's principles as opposed to its practice. For example, there are classes in automotive theory. Basic automotive theory teaches about the different parts of an automobile and how they work. Music theory, then, teaches about the different parts of music and how they work.
Music theory is mysterious to so many people because we experience music everyday, we love music, we pursue music for the pleasure it brings us. And all of these are possible without understanding anything about music. When we delve into the "mechanics" of music, we enter the realm of music theory.