A force play occurs when a baserunner is no longer permitted to legally occupy a base and must attempt to advance to the next base. The defense can retire the runner by tagging the next base before he arrives, though not if the defensive team first forces out a trailing runner. In that instance, the force play is removed and the defense must tag the remaining runners to retire them.
First base tends to have the most force plays, as batters are eligible to be forced out at first any time they put the ball into fair territory and it is not caught in the air.
Watch: Josh Donaldson makes a diving stop and throws to second for a force out.