The Superflat World (超平面 Chōheimen?) is the name given to the collection of locations created for the Game & Watch games. The Super Smash Bros. Melee stage Flat Zone and the Super Smash Bros. Brawl stage Flat Zone 2 are representations of this collection. Both stages are actually "games" being played on a giant Game & Watch. As such, both stages are entirely flat and composed of black lines that form the various graphics needed.
The Game & Watches were handheld consoles with one game programmed into each unit. The stages in Superflat World and the moves of Mr. Game & Watch are all based on Game & Watch games.
In a Dojo update, it is implied by Sakurai in his description of Mr. Game & Watch's role in the Subspace Emissary that good and evil do not exist in Superflat World.