A substitution in football is a player that is replaced by another player from the sideline on the same team. Football teams can only have 11 players on the field of play for a down. Unlimited substitutions are allowed only when the ball is dead. Players cannot be substituted without the referee's permission and all substitutions are finalized by the head coach.
Substitutions occur for a number of reasons, the primary reason being for rest and recovery. Since football can be exhausting for a player, having a backup for him is ideal. This way there can always be a fresh player on the field. Substitutions are also for injured players. If a player gets hurt and has to be taken off the field, there is a substitute who replaces him. Once a player leaves the field, he is not able to return during that game. Other substitutes are on the bench until they are put in the game.