A “pressure” in NFL parlance is when the quarterback is hurried, hit or sacked.

Defenses that pressure the opposing QB more than average allow fewer points and win more games, for obvious reasons. QBs under pressure throw away the ball more and make more mistakes (e.g., throw interceptions).

In DFS, you want defenses (DSTs) that pressure opposing QBs more than average. And you ideally want your QB and receivers to be facing a defense that pressures less than average.