is when an author makes up a setting to go with a character (or sometimes the setting comes first). It doesn't have to be an entire world: sometimes a city or country is all the detail you might want or need to come up with.
Worldbuilding is usually a gradual process. You don't have to come up with a fully fleshed-out world immediately, and you don't have to do any worldbuilding at all to play at Pro and MoonBurnt.
But it's great fun.
For some of the worlds in play at Pro, see World.