It means that you have not acquired sufficient points to remain in the division and subsequently are sent down one. Only so many teams go down, so the amount of points you acquire is always in...
Crashing the center is a defensive term. When a defense crashes the center it means they are sending extra defensive players to blitz through the center of the offensive line.
Playing in the box refers to the players whose job it is to stop the run. The Box is usually a combination of 7 defensive linemen and linebackers. They are usually arranged in either a 4-3, 3-4 or...