What is tailgating in sports?

Okay so I am leaving soon for a tailgate party, but isn't that for people who don't have tickets to the game in the parking lot? And if not, aren't you supposed to bring your own food and stuff to cook? I am just trying to figure this out because- wow, my little brother is noisy. Anyways, Can someone tell me about this and maybe answer the above questions? I'd really appreciate it, thanks!