2nd down in football is the second of four downs in the down cycle. If the offense doesn't reach the 1st down marker on the 1st down, it then becomes 2nd down. 3rd down comes after 2nd down if the offense does not reach the 1st down line on 2nd down. If the offense reaches the 1st down line on 2nd down, they are rewarded with a 1st down.
The distance from the line of scrimmage to the 1st down line varies on every down except for 1st down, which is always 1st and 10 (1st down and 10 yards to the 1st down line), barring a penalty. Yards gained on 1st down transfer to 2nd down. For example, if the offense gains three yards on 1st down, the next play will be on 2nd and 7 (2nd down and 7 yards to the 1st down line). The same system applies to all of the downs in the down cycle, except for 4th down where failing to reach the 1st down line results in a turnover on downs.