An offensive player’s goal is to clear the bases when at bat - that is to make a big hit or a home run so that all members of the team move from each base, ultimately landing home. The phrase originated from the motion of leaving the field, because after recording a run the player moves back to the dugout, and the bases would have been cleared. A grand slam is a rare form of clearing the base because there is at least a runner at every base, thus the hitter scores for all teammates.
