From the Super Mario Wiki
The Bumper in Super Smash Bros.
A Bumper is an item in Super Smash Bros. and Super Smash Bros. Brawl. In Super Smash Bros, it remains on the ground when thrown, damaging players who bump into it. There is also a Bumper floating above the Peach's Castle stage. For unknown reasons, the Flipper took its place in Super Smash Bros. Melee. In Super Smash Bros. Brawl, the Bumper reappears. Bumpers in this game can float in midair when thrown, making it possible to use them as deterrents for opponents trying to recover. Bumpers can also lie on the surface if thrown downward toward the ground.
In Super Smash Bros., if one is thrown and directly hits an opponent, it causes a significant amount of knockback despite causing only one percent of damage. In Brawl, no damage is done, but the knockback is even greater. Additionally, the knockback that the Bumper delivers increases as the target's damage increases.
A Bumper in Super Smash Bros. for Wii U
Bumpers will return in Super Smash Bros. for Wii U with a new design: the outer part is silver instead of blue, and the central part is red with a yellow Super Smash Bros. symbol.
Trophy Information from Super Smash Bros. Brawl
||An item that originally appeared in Super Smash Bros. on the Nintendo 64. After many years, it now makes its triumphant return. Bumpers are used in pinball to send balls flying, and they perform the same role in Smash Bros. They react to anything that touches them, including the people who set them. As they can also be set in midair, using them requires good tactics.