Prologues, like every other scene in a book, should either further plot or character. They're often used to convey important information, such as an event that occurs before the start of a book.
However, usually this information is explained anyway later on in the book, which is why I don't use prologues. But they can add a bit of drama factor to the story, so, if done well, can really help your story.
Another thing to keep in mind is holding the reader's attention. I've heard that you have about three paragraphs to catch the reader's attention. (And about 100+ pages to hold it) A prologue is an excellent tool for this, but this should never be a reason to slack on the
first chapter. If the prologue is interesting but you're first chapter is poorly imagined, you'll still probably get dropped.
Also, if readers are anything like I am, when trying out a book, they'll skip the prologue and go to the chapter one
- Terra