This is a label given to a player on the defense of a football team tasked with one of many tasks, but mostly revolving around stopping the opposition's best player, or best performer. The "rover" position was not used very often in the National Football League, it was more popular in the college game. The idea behind a rover was that a skilled defender would only have to watch the opposition's best player, thereby ensuring they would never go uncovered. The position adapted to one which is tasked with stopping short runs and covering the short passing zone. Sometimes, the rover would be used to watch the quarterback.
This position did not catch on in the NFL because players were too skilled to be only concentrating on one opposition player. The loss of a skilled defender on another player who was not in possession of the ball would put the defense at a disadvantage. The concept of a rover is still used; however, the player is usually assigned to watch a given position or space, rather than a particular player.