Force play

In baseball, any player from the team that is trying to score who is on one of the bases (a "runner") can not be safe on that base anymore if there is another player from their team who has become unsafe at the previous base in the same way; the runner is then said to be "forced" to run. The batter can be considered as no longer being safe at home base (which can be thought of as the base that comes "before" first base) when he has hit the ball into the fair part of the field. A forceout occurs when a player on the fielding team touches a base while holding the ball and a runner at the previous base was made unsafe in this way; that runner is then out.
In other words, a runner is forced to run to the next base, with the risk of being out if an opponent holding the ball touches the next base before them, in only the following two situations:
If, during the play, a runner is gotten out, and there are other runners who are not out and currently forced to run, then any of those runners that reached a base that came after the base(s) touched by the runner that got out is no longer forced to run. This is called "removing the force".