Not what you know about life or history or your interests–but what you know about your story. For example, suppose you have a character who is developing slowly. Much of his past and his motivations are still a mystery, but you do know that he values trust ultimately. Write that down, and build on that information. Because he values trust so highly, does that mean he will never betray anyone–or perhaps he did in the past, and that’s why he values it so highly now?
Or suppose you don’t know for sure when scenes come next in your story, but you have general ideas of what events happens when. Rather than trying to organized this right off the bat, just write down whatever you picture happening at different plot points, and you can add to it or revise it later.
Writing down what you already know about your characters and your story gives you a base to build on, a kind of a foothold. It also keeps you from getting bogged down in what you still have to figure out–because you’ll already have made a start!