If something is behind a thing or person, it is on the other side of them from you, or nearer their back rather than their front. I put one of the cushions behind his head. They were parked behind the truck.
In a place or time that has been passed or left by. Their worries are behind them.
"the children left their books behind"; "he took off with a squeal of tires and left the other cars far behind"; "when he died he left much unfinished work behind"; " left a large family behind"