"Well, the door to the world is--a door," Sora began lamely, not sure how to go about explaining, which didn't stop him from beginning to explain anyway. "It lets you go into the world, but it also lets darkness get in--that's how the heartless travel. So you have to lock the doors to keep the heartless out, but then you can't get in, either, except by other routes." Other, more difficult routes. He began spinning his keyblade restlessly. "Heartless are what happens when a heart is overcome by darkness. They make a heartless and it eats hearts. Nobodies are what's left over once the heartless leaves."

It wasn't the best explanation, but that was basically it. He would really rather get to smashing the whatever-it-was that was guarding the door.