A runner on first base will stand slightly toward second base. This is called leading off. When the pitcher begins his pitching motion, the runner will sprint toward second base. He only has a few seconds to get there safely, because once the baseball reaches the catcher, the catcher will throw it to the second baseman as fast as he can.
If the runner reaches the base before the second baseman can tag him out, the runner is safe and now occupies second base. Attempting to steal a base is a gamble that only the speediest baserunners are willing to take, and often only in low-risk situations (for example, when there are less than two outs).
Sometimes, a potential base-stealing runner is put out even before he has the chance to steal the base. Runners often stand near the base they are currently on, but in the direction of the next base. This is because runners are not required to touch the base when someone is at-bat, so they take advantage by standing slightly closer to the next base. Runners who are planning to steal will try to get as far in advance as they can get, which puts them at the risk of being picked off.
Pickoffs most often occur at first base: Before his pitch, the pitcher will quickly throw the baseball to the first baseman, who tries to tag the potential base-stealing runner with the baseball. If the first baseman tags the runner before the runner touches first base, the runner is out and the pickoff is successful.
Double steals are rare occurrences in baseball. They may refer to two (2) baserunners stealing bases in the same play (either simultaneously or with a slight delay between them), or a baserunner stealing two (2) bases in one play.