From BR Bullpen
A pick-off is a play in which a baserunner is put out when straying too far from the base he is occupying. A pick-off cannot happen on a batted ball; it either takes place before or after a pitch is thrown.
The most common form of pick-off occurs when the pitcher throws to a base (most commonly first base) from which the baserunner has taken a lead. The runner must return to the base before a tag is applied by the fielder receiving the throw. If he fails to do so, he is out, with the putout credited to the infielder and an assist to the pitcher. Alternatively, the runner may decide to try to run to the next base - if it is unoccupied - instead of returning to the base he is occupying. If he is successful in reaching it before the ball can be relayed there, he is credited with a stolen base; if not, he is out on a caught stealing with the pitcher receiving an assist. Most balks called are the result of improperly executed pick-off attempts.
The second form of pick-off occurs when, after catching the pitch, the catcher notices that a baserunner has ventured far from his base. If his throw to that base arrives in time for a fielder to tag the baserunner before he can return to the bag, the runner is out on a pick-off.
A pick-off is considered to be a baserunning mistake, even if it not usually captured in individual statistics as it is not a caught stealing. Inversely, it is also a clutch play for the defense, which can both record an out and erase a baserunner in one stroke, without allowing any other runners to advance.