What is music theory?
Music theory is many things. Lots of things! First we have sheet music. Not so much to play from notes, but more to learn to understand how the note system is structured, so that it becomes easier to use the notes when playing.
There are also many other characters in the music than notes, and you also have to get to know them gradually. There are signs that show whether to play strong or weak, play the notes closely together or separately, play slower or faster, and many other things.
After playing for several years, you can also start learning about how the music is made, how a chord is built, how chords fit together theoretically and much more.
In the music school’s theory teaching, we do not go into the most advanced things, but you can get a lot of knowledge from following the teaching and exercises.