What is Musical theory?...

Music is a very large subject.  This is an introduction to music theory.  After mastering the basics you may want to pursue advanced theory as it relates to your field of interest.  A trumpet player into jazz or a guitarist interested in world music or a vocalist studying early music will all delve into different facets of music theory.

You might ask why theory is so important.  Theory is a bit like language and grammar.  If you are going to communicate with other musicians you need to speak the same language.  If you want to learn new music, having the ability to read music opens up a whole new realm of possibilities.  You certainly can learn by ear and I know people who can only do that.  But the time it takes to learn by ear is reduced exponentially if you learn basic theory and can read music reasonably well.  Even if you do not become a good sight reader, being able to plunk or pick out basic melodies and chords will get going in the right direction much faster.