The bumper is basically a rubber protector which runs around the side of your device and slightly raises the back of the device to help prevent it from getting scratched.
The bumpers have been out of stock for some time now, but they can now be ordered for $19.99 with delivery in 3 to 5 days.
The bumper is available in black, and is expected to arrive in white to match the upcoming white Nexus 4. You can order one for yourself by following the source link. Considering it got sold out before, you may want to get your hands on it quickly while stocks last.
source - Google Play