This is a phrase used in baseball to describe a play in which a batter literally hits the ball behind the runner. For example, if a runner is on second base, and the hitter hit the ball down the first base line, it would qualify as hitting behind the runner. This is a favorable place to hit the ball, as it creates more distance between the ball and the destination of the runner. However, it does have the downside of putting the hitter at greater risk of being put out.
There are many hitters at the Major League level capable of controlling where they hit the ball. On these teams managers may request a player hit the ball down the first base line, or as far as up the middle, in order to advance runners. Although hitting behind the runner typically benefits the lead runner, there are instances where the defense could take advantage. If the runner is on second base and the ball is hit to the second baseman, positioned between first and second base, there is potential for a double play.