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."
I look forward to tailgating this fall at a college football game. Here in the South it is part and parcel of the culture. There are many who never see the game; they stay out by their cars and drink and eat and dance the shag, which is very Southern. Tailgating also occurs at horse races. The Carolina Cup in Camden, South Carolina, is famous for throwing the state's larget outdoor cocktail party in this fashion. People line their cars in th eopen field across from the racetrack, set up a table or use the back of the car to spread their drinks and wonderful hors d'oeuvres. Some bring linens and silver candlesticks and silver drinking cups in which to enjoy their beverages. It is a vestige of the former fabulous glory that is the South, which has, in fact, risen again, ya'll.