It's so much more than that. I'm not a tailgater, but this is something near and dear to many people's hearts. Done especially outside football stadiums on game day, not always in parking lots (but sometimes). For college sports it's often done on/near the campus quad or some other nicer green space.
Basically it's a picnic lunch with your friends before the game. Some people bring huge grills and do up the meal really fancy; others just have burgers/hot dogs, that kind of thing. Usually a bit of drinking going on as well.
So called because you usually flip down the tailgate of your (originally station wagon) SUV or truck, and put the grill, cooler, etc. on it. Or sit on it.
In my neck of the woods, where tailgate parties are unknown, tailgating refers exclusively to a very rude and inconsiderate, not to mention dangerous, driving technique (actually "technique" is too generous a term), in which someone drives too close to the car in front, often in an attempt to intimidate. An expression of road rage in many instances.
Good point, frindley and rt. I was shortening the expression ("I'm not a tailgater") without thinking of the other, more common meaning of "tailgate." Sorry to confuse. I meant, "I'm not a tailgate party-er."