A first down in football is when the offensive team successfully advances the ball at least ten yards within their set of downs, which resets their count of downs and allows them a new series of attempts to continue advancing the ball. Specifically:

• The offense has four downs (or attempts) to move the ball ten or more yards forward.
• If they achieve this ten-yard gain within those four downs, they earn a first down, which means they get a fresh set of four downs to try to advance another ten yards.
• If they fail to advance ten yards after four downs, the ball is turned over to the other team.
• The first down is crucial because it allows the offense to maintain possession and continue their drive toward the opponent's end zone to score points

In summary, a first down marks the successful completion of a series of plays that move the ball at least ten yards, enabling the offensive team to keep their possession and keep trying to score