From the Super Mario Wiki
A Bumper is an item in Super Smash Bros., Super Smash Bros. Brawl, and Super Smash Bros. for Nintendo 3DS / Wii U; in Super Smash Bros. Melee, the Bumper is replaced by the Flipper. In Super Smash Bros, it falls to the ground when thrown, causing only one percent of damage to players who bump into it and causing weak knockback. As of Brawl, Bumpers can also float in midair when thrown (making it possible to use them as deterrents for opponents trying to recover) and only damage players hit by the initial throwing. They no longer move if bumped into, although the knockback is far greater, and it increases as the target's damage increases.
In Super Smash Bros., there is also a Bumper floating above the Peach's Castle stage; this Bumper retains its original appearance and damage properties in the stage's DLC return in Super Smash Bros. for Nintendo 3DS / Wii U.
Bumpers return in Super Smash Bros. for Nintendo 3DS / 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.
Super Smash Bros. Brawl
Super Smash Bros. for Nintendo 3DS / Wii U Trophy