Prologue

That just about says it all! :)

If you put Prologues in the Enter search terms box up there on the right, you'll see this question has come up regularly. Definitely a love 'em or hate 'em thing.

The purpose of a prologue is to be an introduction, or a fore-warning, or a threat or a promise. It may not be where the main narrative starts; it may discuss things that happened before the events take place, or even in some cases after they have happened. It may exist to explain why the characters act as they do, or to introduce a voice that has no active part to play in the ensuing work, but whose deeds have sparked those events.

There are all sorts of reasons to have one, but ultimately it has to add to the novel.