Base runners are required to advance the bases in order of first base, second base, third base, and home plate to score a run. This is called the circuit of bases. Base runners do not have to complete the circuit all at once. In fact, they can secure their position on base when the ball is live. However, only one base runner can occupy a base at a given time. This will be important as we begin our discussion of force plays.
Force outs, also known as force plays, happen when a runner must advance to the next base because a runner behind him is approaching his current base, and he is put out as a result. The base runner is force to leave his previously occupied base because the batter becomes a runner and must occupy first base. This creates a domino effect forcing all base runners to advance.
For example, if a batter hits a ground ball and there is a runner on first base, that runner must advance because the batter-runner is approaching first base, and they cannot occupy that base at the same time.
If the second baseman has fielded the baseball, he can simply step on second base before the runner (who was previously on first base) reaches it, and that runner is out. Since the original runner was forced off his previous base to advance to a base that had just been tagged by a fielder, that runner is considered to be forced out.
Sometimes, batter-runners reach first base on what is called a fielder's choice. This occurs when there is a runner already on base, the batter hits a ground ball, and the fielders decide to put out the pre-existing runner at his next base rather than the batter-runner. Therefore, the batter-runner reaches first base safely. Batters who reach first base on a fielder's choice are not credited with a hit, since they would have otherwise been out if the runner had not been there.