In football, the ball is always moving on the field. The offense is trying to bring the ball closer to the opposing team's end zone to score points. The movement of the ball can be described in yards.
Football Loss of Yards
Football No Gain
Football Chain Crew
The chain crew is a group of referees responsible for marking the line of scrimmage and distance to the first down line after every down. They are called the chain crew because they carry a massive to chain to measure the exact location of the ball on the field.