Your game should, by now, be playable. Perhaps there is only a single, empty room to run around in. Maybe you've got a title screen and some movement code but there's nothing much to see or do.
Consider yourself in great stead if you've reached that point. The next step is to flesh out your game world—to add enemies or puzzles or levels, or to create "gameplay" that your basic engine is capable of supporting.
Up to this point you may have, for example, decided to create a platformer game such as Super Mario Brothers. If you were smart, all you worked on was running, jumping, and collision detection so you won't fall through the ground and can't run through walls.
The next step is to create a bunch of levels that are fun to explore. There are two approaches to doing so: hand-craft your levels (which will necessarily be few in number) or let your computer help you create more content.
To create levels for your game, you may...