A returner in real football is someone who returns a kicked ball. That could either be on a punt or a kickoff. The returner is apart of the special teams unit.
In fantasy football the returner is grouped into a unit along with all of the defense. One way defense/special teams earn points are through touchdowns. That could either be on a defensive score or a return touchdown. This is the only way a returner can earn their points, as they are not awarded points for gaining yards. A kickoff or punt return touchdown goes towards their team's defensive and special team's point total.
Many returners are actually players that play another position as well. It could be a wide receiver or a running back that is doubled up as a returner due to their speed or quickness. So what that means is that you could start a wide receiver in the receiver slot and still earn points for their returns.
Given the dual status of some players it might be a good idea to draft a player that does both. If you draft a wide receiver who also returns punts, that can give you an extra couple points here and there. You never know, it could be what decides a crucial game for you!
However it is not recommended to draft a player that exclusively returns the football. Even the best returner will not produce enough points to make this move worth it. The league leaders most years have around four return touchdowns, or 24 fantasy points. That comes out to being about 1.5 points per game, not a worthwhile investment.