In baseball (and softball), a grounder is a ball hit so that it rolls or bounces along the ground, not flying high in the air. Fielders must pick it up and throw. It is everyday game talk, not a fancy word; commentators say it all the time. The fuller phrase is ground ball; grounder is the shorter, very American form. It is not a person, and it has nothing to do with a parent grounding you, or with electrical grounding. Though actually, outside baseball most English speakers would not use it — they would just say the ball rolled along the ground.