A 1st down in football is the first scrimmage down in a series of four downs. The offense has four chances to advance the ball 10 yards and pick up a new first down, otherwise, the other team gains possession of the ball. If the offense reaches the 1st down line, they are rewarded with a fresh set of four downs. The 1st down line is represented by a yellow line that stretches across the field.
The 1st down is a part of the down cycle, a series of four downs in which the offense attempts to gain 10 yards and reach a new 1st down marker. The other downs in the down cycle are 2nd, 3rd, and 4th down. When the offense is faced with a 4th down situation, they will likely elect to punt the ball away to the other team if several yards are still needed for a first down. If only a few yards are needed for a 1st down or the team on offense is down with little time left in the game, they will run a play at the risk of losing possession where the play ended should they fail to reach the 1st down marker.
Offenses generally call running plays on 1st down. Running the ball on 1st down not opens up passing plays by forcing the defense to honor the ground game, but it also helps the offense to pick up a few yards so that shorter passes can be used to pick up a 1st down. For example, a very successful run on 1st down (gain of 5+ yards) would lead the defense to believe that the offense will attempt a running play again, making for a relatively easy pass completion on 2nd down. Passing has become a focal point in the modern era, making 1st down plays incredibly hard to predict.