When a fielder does not have the ball and is not in the act of fielding a batted ball, an obstruction occurs if that fielder impedes the progress of a base runner. The umpire makes an obstruction signal by extending the left arm to the side with a fist. If the impeded runner is put out prior to reaching the next base or a subsequent base that the umpire believes she would have reached if the obstruction had not occurred, then the out does not count and the umpire will put the runner on the base she would have reached absent the obstruction.
It is very important to understand that the ball remains live after obstruction occurs and that the runner must try to advance in order to get the base to which she is entitled. When the umpire lowers his or her arm, the runner is no longer protected, either because she has arrived at the base she would have reached or she has failed to try to advance.
What does it mean for the player?As fielders, players have a hard time learning to stay out of the way of runners. We see obstruction called in almost every game, and the most common obstruction comes from a fielder standing on a base without the ball when the runner reaches the base. If a fielder is playing directly between two bases when the ball is pitched, she needs to be prepared to move out of the way of a base runner if the ball isn't hit to her. After a ball is hit, if there is no play at her base, then she needs to stay away from the base and out of the runner's way.
A runner (as well as the coaches) should look for an obstruction signal by the umpire if she's impeded, and she should continue running to the next base even if a tag is likely. As long as the umpire is making the obstruction signal, the runner is protected in advancing.