A tailgate is the hinged opening at the back of a pickup truck. And as a verb, to tailgate is to drive too close to another vehicle.
It's dangerous to tailgate, because you might crash into the car in front of you if it stops suddenly. This verb gets its name from a vehicle's tailgate. If you're too close to the tailgate of the truck in front of you, you're tailgaiting. Another informal use of tailgate is "party in the back of a truck," especially in the parking lot of a sports arena: "Let's tailgate before the big game tomorrow!"