Did Christ Die For The Whole World?

"And he is the propitiation for our sins: and not for ours only, but also for the sins of the whole world."

This verse puzzles some believers. Is it saying that Christ saves everyone? Or is it saying that Christ's death made it possible for everyone to be saved, but that you have to do something to become saved? Or is it saying something else entirely?

Understanding this verse and I Jn. will keep us out of all sorts of dangerous heresies.