A down is a term used to explain the allotted amount of plays a team has left before they lose possession of the ball in gridiron football. In the NFL, teams have four downs to attempt to advance ten yards or more on the field, or reach the endzone and score a touchdown. Advancing the required distance will result in a first down. Not advancing the required distance will result in advancing to second, third, or fourth down.
Not advancing the required distance on fourth down will result in the offense team losing possession of the ball, and the defense team being able to play offense on the spot that the previous play ended on. This is usually a negative for the offensive team, and this is made worse if the offense team was very close to their own endzone, as the opponent team starts next to the endzone and has a good chance to quickly score a touchdown with a few plays. Teams are more likely to punt if they are close to their own endzone or attempt a field goal if they're close to the opposing team's endzone.
In Madden NFL the required distance to advance to earn a first down is marked with a yellow line, or a red line on fourth down.