2. Something that the majority of modern music artists lack knowlege of
Person 2: "Whats common time?"
Person 1: "Oh great, I'm jamming with a moron."
You look like an asshole when you don't know music theory.
Music theory mostly deals with the earliest music ever written on paper to music written in the late 1800's.
Specifically, music theory is made up of laws, rules, labels and other insights regarding Western music (particularly its harmonic tendencies), pioneered by a man named Rameau.
Music came first, theory came after.
Also, something barely any musician has nowadays, and those who have none say that you "don't play from your soul" if you have knowledge of music.
These people are dumbasses.