The stopper is a player in football (soccer) who is either considered a defender, or a defensive midfielder, depending on the formation. He will either play at the tip of a diamond or triangle defensive formation, or as the rear of a diamond or triangle midfield formation. He does not mark (guard) a specific man (although he will probably end up with the same offensive midfielder much of the time) and instead watches the entire center midfield (about 40 out of 70 yards on an average sized field) picking up (moving to guard) any player that makes a run down the middle.
His name derives from the fact that he waits to "stop" these attacks, and this position is also sometimes referred to as a Rover because he roves around the "backfield" (area behind the offensive players) looking for a man to pick up. His position is an important one that determines the safety of the entire area in front of the goal. If he does not do his job properly the 18-yard box (that big white rectangle in front of the goal) will be flooded with unmarked players just begging for a head ball. Also, he will leave men exposed to take rebound shots on the top of the "18" which makes for an easy defeat in a match often determined by only one goal.