Understanding music theory means knowing the language of music. The main thing to know about music theory is that it is simply a way to explain the music we hear. Music had existed for thousands of years before theory came along to explain what people were trying to accomplish innately by pounding on their drums. Don’t ever think that you can’t be a good musician just because you’ve never taken a theory class. In fact, if you are a good musician, you already know a lot of theory. You just may not know the words or scientific formulas for what you’re doing.
The concepts and rules that make up music theory are very much like the grammatical rules that govern written language. Being able to transcribe music makes it possible for other musicians to read and play compositions exactly as the composer intended. Learning to read music is almost exactly like learning a new language, to the point where a fluent person can “hear” a musical “conversation” when reading a piece of sheet music.