The bumper is an item original to the Super Smash Bros. universe, and only missed the series once with Melee, which was rather odd. It's based on the bumpers of pinball machines.
While it doesn't deal a lot of damage when someone is hit by it, it has a huge push-back to it.
Which make it rather easy to get knocked off the stage with. Any smart player knows to be careful around one of these.
Throwing one near the edge of the stage is a good way to prevent people from getting back up. Though I have found most people usually find a way over or under it. But it is one of my favorite items in the series and in Ultimate.