A force out in baseball occurs when a player is no longer allowed to occupy the base but must advance to the next one. This can happen when a runner is approaching his current base, and he is put out.
The baserunner is forced to leave the base he has occupied. This is due to the fact that the batter becomes the runner and must occupy the first base. The concept of this rule is based on the idea that two baserunners can’t occupy the base at the same time.