You get to draw on one tile in a large picture. You're only able to see edges of tiles next to yours. When a tile is marked with a questionmark, that means it has not been taken yet. You are free to draw whatever you want, but it is encouraged to try to make your piece intergrate with what is already there. Once you click submit, your tile will be saved and no longer be editable. You can view the final result in the gallery.