This is a baseball term which describes the action of a pitcher or catcher attempting to, or actually throwing out a base runner attempting to steal a base. In baseball, a player already on base can steal the next base by running while a pitcher is in motion delivering the pitch. To prevent a player from doing so, a pitcher or catcher will throw the ball to the first or second baseman respectively, who will make an attempt to tag the player out. Many pitchers have "pickoff" moves they use to disguise their pitching motion. Many runners will take a lead off the bag, putting them at risk of being picked off, but also giving themselves a head start if they intend to steal. The size of their lead is often determined by the speed and accuracy of a pitcher's pickoff move. A pitcher will throw to first numerous times during an at-bat if there is a particularly fast runner on base.
There are other reasons for using a pickoff other than attempting to get a player out. There is the previously mentioned preventative technique, which is used to convince a base runner that that the defense is aware of their position and will be watching their movements closely. Another use of the pickoff move is to analyze the offense. By making a pickoff move, a pitcher effectively fakes out the batter as well, who may show that they are intending to bunt. The last reason a pickoff move may be used is to stall for time. This last reason is primarily to provide a little extra time for a relief pitcher to warm up before coming in the game. This has no real effect on the game other than making it last longer. However, the primary use of a pickoff move is to get a player out, and it may be performed by a pitcher or a catcher.