This is any offensive or defensive player who lines up the furthest in any direction.
This can refer to many positions in the game of football. Often times on the defensive side of the ball you line up with a right end and a left end. On the offensive side of the ball this typically will refer to the right tackle or left tackle, who both line up on the end of the line. The tight end also will line up outside of them putting him at the end. The Houstan Texans' Mario Williams plays on the end on the defensive side of the ball. He was a first round pick by the Texans.