American baseball. Which means, it's in play; it's acceptable; we can work with that.
It gets its meaning because a hit inside the ballpark is a legal play and the runners can advance around the bases.
Baseball was broadcast over the radio so when the announcer said, "It's a hit!" the audience waited to hear if the hit was fouled (outside the field of play, do-over), a fly ball (an out) or inside the ballpark (runners advance and fielders try to "throw them out" by getting the ball to the base before the runner arrives).
Or a hit could be a "homerun" which means the batter hit the ball so hard it went over the back fence and the batter gets to score.