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.
When the offense moves the ball closer to the first down line and the opposing team's end zone they gain yards. This is a called a gain.
Football Loss of Yards
When the offense moves the ball further away from the first down line and the opposing team's end zone they lose yards. This is a called a 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.