As a noun, jeer is the act of scoffing, taunting, or mocking. Think of it as an anti-cheer. If you offer cheers for the visiting team and jeers for the home team, you might not be too popular in the stands.
As a verb, jeer means to laugh at in a mean way. Even if you did forget the words to the national anthem, it wasn't polite for the crowd to jeer. But don't feel too bad: the last singer remembered all the words and they jeered her anyway! Tough crowd.