What is a touchback?  If the ball is kicked into the end zone during a kickoff or punt, you might see the kick/punt returner take a knee to signal for a touchback. Taking a knee is exactly what it sounds like, the player who catches the ball gets down into proposal position in the end zone. Once a touchback is called, no one can tackle the kick/punt returner and the offense will start its first down at its 25-yard line.

Why did they start this rule?  The kick/punt return is one of the most dangerous parts of the game. Defenders are running towards the kick/punt returner at full speed from far away, which can lead to serious injuries.  The idea is to make touchbacks more attractive to reduce these dangerous tackles. With the 25-yard incentive more kick/punt returners will make a calculated decision to take a knee instead of running out of the end zone to risk advancing fewer than 25 yards.

In the NFL, the trend has been to move the touchback line closer to the opposing end zone.  For example, during the 2016 season, the NFL is temporarily moving the touchback to the 25-yard line from the 20-yard line to test it out whether this reduces injuries.  

Critics say that the rule change won't make a big difference because kickers and punters will just get better at targeting their kicks. They will try to aim their kicks so that the footballs drop between the end zone and 25-yard line.  That way the kick/punt returner won't have the option of signaling for a touchback, because you can only take a knee if the ball is caught in the end zone.