What is music theory?

A framework that gives our human imagination the widest possible scope for emotional expression through music.

:lol:

It's the language of music so you can speak to other musicans and communicate effectively.

For example, if I say we're gonna play a one-four-five 12 bar blues progression in the key of E, would you know what to do? Would you know how to solo over the top of it?

If you know a particular song in one key and the band wants to do it in another key, could you do it on the fly?

Do you know how chords are built and could build one particular chord in mulitple places on the fretboard?

If you are writing a song and have a chord or two in your head, would you know what other chords might work with the ones you already got?

It's the language of music so you can speak to other musicans and communicate effectively.
It's as much the grammar of music as the language. It is the condensed result of hundreds of years of practical music as to what works best with what.
It is not an absolute law, that can not or may not be broken.

If you are looking for a quick application for music theory, research a little about the Circle of Fifths (it is also described in the forum post below) then go to the following website:

http://didgeridootribe.org/didgeridoo/index.php
Type in the words "Playing in Harmony Together" in the Search text box and hit Enter. Have a good read with this post. So far, it is the best concise and useful description of a simple way to figure what keys to play in when jamming with others.

It's a standard language or grammar, as Greybeard said, but it's also a way of organizing things. This is from the introduction to my theory book:

Just as a painter mixes colors to make a specific hue, musicians combine sounds in order to create sonic impressions. A painter might place his colors at random on his palette, but organizing the colors - always putting the blues and reds in the same place - aids in finding exactly the elements needed to create a specific shade.

Music is much the same. If your knowledge of the elements of music isn't well organized, you can still create the shades of sound you want, but you'll have to hunt around for the elements. If you understand music theory, the sounds you want are organized; you know which sounds go with certain chord progressions, or which notes will create a specific mood. Understanding music theory will give you a foundation to work from. It will also open your eyes (and ears!) to possibilities that you might never have found without the tools you'll gain through studying theory.