"Stemming around" is a tactic used by defensive back football players that consists of moving around before the snap after giving the other team's offense the impression of having settled the defensive positioning. This can lead to a misread of the defense, and hopefully a mistake made by the opposing team.
The tactic is used to trick the opponent's offense, and most importantly, the opposing team's quarterback. By doing so, the defense seeks to disrupt the offense's play and force a mistake, leading to an interception, fumble, or a sack on the quarterback.
Football rules limit the mobility of the offense before the snap, and so Defensive Coordinators understand the advantage that stemming around can bring. That way, it is very common for defensive plays with stemming around to be called. There are many plays that involve stemming around. One of the most popular ones aims to give the quarterback the impression that man-to-man defense is being played, when in reality, zone defense is being played.