Like Forge in Halo 3 or various map-makers in other Wii games (Elebits or Dewy's Adventure), this allows you to make your own custom stages and play them at any time.
The stages are fully customizable, allowing you to place every block and part found in the game. You can choose the background, basic size (small, medium, large), and even the music. You can do just about anything: flip parts, shrink/enlarge parts, change the color.
Really, this gives infinite replay value to the game. Even more so since you can send and receive maps from your friends over the internet.
For a more in-depth look, see the Stage Builder section.
