You can:
- Move and change the radius of the spawn point.
- You can adjust the water level
- You can change the skybox
- You can stretch and move some objects, but stretching bricks will crash the game
- You can delete bricks
- You can edit the terrain, but the changes won't show on anyone else's client
Those are some of the things you can do, but I don't know a whole lot about it.
You must be the host of a game for the editor to work.