In a standard flex league, the flex spot can only be filled by a running back, wide receiver, or tight end. A Superflex league opens that spot up to quarterbacks too. Since quarterbacks tend to score the most fantasy points, most managers start two quarterbacks every week, which changes how the whole league is valued and drafted.
The biggest impact shows up on draft day. Quarterbacks fly off the board much earlier than in one-quarterback leagues, and the top options at the position often go in the first two rounds. Waiting on quarterback becomes a much riskier strategy, because the drop-off after the starters are gone can be steep. Quarterback depth also matters more during the season, since a bye week or injury can leave a manager scrambling.
Superflex leagues usually also raise trade activity, because managers who stockpiled quarterbacks can deal from that strength to teams in need. Some leagues pair Superflex with other settings, like point-per-reception scoring or tight-end premium, but the core idea stays the same: the flex spot is open to every skill position and the quarterback.