What does Avant garde mean?

In French it means the soldiers that go before the others. "Avant" means "before" in French. The idea is that the avant garde are scouts intended to explore terrain before the main army gets there. The expression is used a lot in the arts where it means artists who are dealing with extremely experimental art forms that the main body of artists haven't even thought of yet.
Another aspect of the metaphor is that the scouts quite often go in the wrong direction and get lost or destroyed or both. This happens with avant garde artists as well.