The most common stack pairs a quarterback with one of his pass catchers, like a wide receiver or tight end. When the quarterback throws a touchdown, both players score points on the same play. That connection can produce big weeks when the offense clicks, which is why stacks are popular in daily fantasy sports and best ball leagues, where a few huge weeks can win you a tournament or a season.
Stacks come with a tradeoff. When the offense has a bad game, both players likely flop together, which hurts your weekly consistency. A popular compromise is a 'correlation-lite' approach, such as pairing a quarterback with a running back from the same team, since the running back often benefits when the offense scores even if the quarterback throws to someone else.
In season-long leagues, many managers avoid stacking their quarterback and best receiver from the same team because it concentrates risk. In playoff formats or daily fantasy, stacking becomes more valuable because you are trying to maximize your best-case outcome for a single week rather than stay steady all season.