What is "inside-the-park home run"

Home run \Home" run`\, n. (Baseball) a complete circuit of the bases made by the batter without being put out and without an error on the play; also, the hit on which the batter makes such a circuit; a four-base hit.

Note: Most home runs are made when the batter hits a fair ball out of the playing area on a fly, when the ball cannot be played by the defending team; if a batter circuits the bases while the ball is still inside the playing field, it is an inside-the-park home run.

In baseball parlance, an inside-the-park home run is a play where a batter hits a home run without hitting the ball out of the field of play.