The term "stacking" in fantasy football refers to fantasy owners using multiple players from the same team in their fantasy lineups, hoping that the team will perform well collectively and yield double the points.
One of the more popular approaches with regards to stacking is to set your lineup with a quarterback and wide receiver from the same team. Ideally, owners would like to stack the quarterback with the team's top receiver, hoping that they will earn points for both passing yards/passing touchdowns and receiving yards/receiving touchdowns.
Another popular stacking strategy is to stack one team's quarterback and running back in the same lineup, so that the entirety of that team's offense is accounted for from a fantasy perspective. In other words, success on both running plays and passing plays will yield points for the owner's fantasy football lineup.
Stacking is high risk strategy with the potential for great rewards. By using two players from the same team, the owner is essentially going all-in and relying on that team's offense to perform well as a whole. If the offense plays well, the owner benefits from nearly double the fantasy points that would've been achieved without stacking. If the offense falters, however, the owner suffers from poor output from not just one, but two players.