The principles/guidlines of classical, real music. Modern musicians tend to neglect music theory or have little knowledge of it. However, most knowledgeable musicians have a basic idea of theory. (Steve Vai and Yngwie Malmsteen to cite two)
Person 1: "Hey, play me a Em, FM, and GM in common time so I can put together a solo using the Em scale."
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.