A touchback in football is a ruling on a play when a ball is dead in the endzone and the defensive team has possession. The ball has to have traveled into the endzone via the offensive team.
Touchbacks can result from the following:
The ball is left at the 25-yard line following a touchback. The team who was on defense and gained possession of the ball will start a drive on first down.
Originally, the ball was placed at the 20-yard line after touchbacks. In 2016, the NFL moved touchbacks up five yards to the 25-yard line. The change provided an incentive to the receiving team to take more touchbacks rather than trying to return kickoffs.