A baserunner in baseball is a term used to describe a player on the offense who is positioned on a base. His goal is to advance between bases for an opportunity to score a run if he completes the circuit touching first base, second base, third base, and home plate.
As soon as the offensive player successfully bats the ball, his or her primary objective is to now run to first base before being tagged out by a member of the defense. A tag-out occurs when a defensive fielder manages to tag the baserunner with the ball before the baserunner has reached his or her base.
There can only be one baserunner on each base at any given time. Hence, this means that if player A is currently up to bat, player B is on first base, and player C is on second base, and if player A succeeds in batting the ball and runs towards first base, both player B and player C will then be forced to run to their next respective bases. However, if player A fails to successfully bat the ball, player B and player C can remain on their respective bases. If player B still wishes to advance onto second base however, he or she may only do so if player C chooses to advance onto third base.