From BR Bullpen

A pitch-out is a pitch thrown deliberately outside the strike zone in order to attempt to prevent a stolen base. The purpose of the pitch-out is to allow the catcher to catch the pitch standing up and away from the batter, thereby giving a much easier and faster throw to the base being stolen.

Even though a pitch-out is usually far outside the strike zone, the batter may still attempt to swing in order to make the catcher's task more difficult. In doing so, he must be careful not to step out of the batter's box or commit interference, as this may result in the baserunner being called out. For his part, the catcher must make sure he does not leave his box before the pitch is thrown, or it will result in a balk. Unless the batter swings, a pitch-out is always a ball; as a result only one or two can be called in a single at bat, or the batter will end up with a very favorable count or a base on balls, thus negating much of the purpose of the strategy.

A pitch-out usually differs from an intentional ball in that the pitch is always a fastball. It can be called either by the manager or a coach from the dugout, or by the catcher. The usual sign for a pitch-out is a closed fist pointed away from the batter.