This is a baseball rule which restricts the actions of fielders in the base path while an offensive player is running. If a defensive player hinders the advancement of a runner, the fielder is considered an obstruction and will be in violation of the interference rules. The only instances where a fielder does not have to allow the base runner freedom to move between the bases is if the fielder is actively fielding the ball, or is in possession of it. There are two potential outcomes when a player gets in the way of another. If the runner in question is being acted upon by the fielders, meaning they are actively attempting to get the player out, then the ball is immediately ruled dead when the obstruction occurs. If the runner in question is not being acted upon, then the play continues and the runner is then awarded the base the umpires believe the runner would have reached had they not been obstructed.
Although obstruction is very similar to interference, the rules governing interference are must more complex and cover many more scenarios. Furthermore, it should be noted that some broadcasters, fans, and even players, will sometimes mistakenly refer to obstruction as interference. In the case of obstruction, the defensive player does not need to have the ball and there does not even need to be any physical contact. If the umpire believes the defense impeded a runner from advancing illegally, they will call obstruction.