This is a line on either side of a football field which runs parallel to the sideline. In the National Football League, it is positioned six feet from the sideline. This line denotes how close non-active players are allowed to get to the field. The reason for such a line is to ensure inactive players are not confused for active players during a live play. It has the added benefit of making room for coaches and other team staff to get closer to the action while also providing a space for players to rest on the bench.
This line runs parallel to the sidelines until the thirty-yard line at either end of the field. Players are not allowed beyond the area encompassed by the restraining line and its end-lines. If a player crosses either line they will be asked by game officials to move. If a player disobeys they may be fined by the league, but more often they will be asked by coaches to move.