false start, ineligible receiver, or not enough players on the line in football. It is one of the more important rules to keep the game flowing. It is pretty easy to understand if you know anything about soccer.
It is hard to be interested in something that you don't understand, choose to learn or really don't care about. Hockey is probably awesome, but I don't care. Same with LAX (my son played and I watched games, but don't care).