One team gets the ball. They line up on the field. The team with the ball tries to get the ball into the "end zone" that they're facing. The other team tries to stop them. They get four plays to do it. If they get more than 10 yards they get another four plays. If they fail, the other team gets the ball and they do the same thing, going the other way. There are a bunch of other little plots in the story, but that's basically what's going on.