A bumper is the curved bar across the front of a car that protects it from getting dented or scratched during minor accidents. If you bump into your mailbox, you may damage your car's bumper.
A car's bumper is designed to absorb as much impact as possible, protecting the more important parts of the car. A bumper car at a carnival is made to be slammed into other bumper cars — the bumpers help keep riders safe from injury, while the cars travel too slowly to cause damage. The earliest bumpers, in the 1830s, were on the front of trains. The phrase "bumper-to-bumper traffic" means an unusually dense traffic jam.