What does 'working out of the stretch' for a pitcher mean?

When there are no runners on base, pitchers take a "full windup", which usually entails a lot of motion with the legs and arms; it takes a relatively long time for this windup to take place and the ball to be released. When there are runners on base, they are allowed to steal the next base and could easily do so if the pitcher takes his full windup. So to counter-act that, pitchers "work out of the stretch," which means they stand with their back foot parallel and adjacent to the pitching rubber. After getting their signal from the catcher you will see them bring the ball to their glove and come to a stop before pitching. From this position they can quickly throw to the base where the runner is to make sure they aren't getting too big of a leadoff. And when they do pitch the ball they just take a fairly quick step forward and fire the pitch. This step is much faster than the rocking motion they use when in the "windup" and therefore there gives much less time for the runner to steal a base. Many relief pitchers will always "work from the stretch" even without runners on base. Since they come into the game with runners on base often, trying to keep consistent with two different pitching deliveries isn't worth it.