This term refers to the rule in baseball whereby a batter is allowed to run to first base (or farther) if the catcher drops the ball after a third strike. The batter immediately becomes a runner once the ball is dropped, so the catcher must either tag the player out or throw the ball to first to get the player out on a force at first base. If there is one out in the inning, first base must be empty in order for this rule to apply. If there are two outs, first base may be occupied and the rule will still apply.
The rule is most often used in higher levels of competition, such as Major League Baseball, and not as often in more junior leagues. The rule is believed to come from the notion that the defensive team must correctly and completely execute the out in order for it to be recorded. In the case of a dropped third strike, also known as an uncaught third strike, the pitcher is still credited with a strikeout even if the player makes it to base after the catcher drops the ball. For this reason, it is possible for a pitcher to record more than three strikeouts in a given inning.