Music theory helps you to understand how music "works" - how notes go together to create characteristic harmonic sounds. Music theory is the "math" behind how notes are named, how scales and chords are formed, and how each of the pitch elements in music are combined to form the sounds you recognize as music. Every style of music uses a particular set of musical elements, so understanding music theory helps you better understand the makeup of your favorite types of music.
If you want to be able to play by ear, compose music, or improvise, then understanding music theory is absolutely essential. Understanding music theory helps players to organize and categorize sounds that are perceived aesthetically as "happy", "sad", "dark", "mello", "sweet", "jazzy", etc. Whatever your intention, learning music theory can help to remove the mystery about how sounds are organized on every instrument, and why certain note combinations are perceived as "musical".
The lessons on this page make up a complete study of fundamental music theory. Applied concepts - playing by ear (figuring out the chords to songs), harmonizing melodies, transposing songs, lead guitar soloing, song writing and music composition, etc. - are covered in the other sections of this site. Approaching any of these other topics requires an understanding of some or all of this knowledge. The lessons on this page are collected to provide a fundamental knowledge of all musical concepts required to form other skill sets.
Please read these articles in order (from top to bottom), as they build upon one another progressively.