A ballpark is the area where baseball is played. When you go to the ballpark, you can sit in the stands, eat a hot dog, and cheer for your favorite team.
Ballpark is an American word from the late 1800s, originally a shortened form of baseball park. The ballpark is familiar to baseball fans, their equivalent of a football stadium or a tennis court. Colloquially, you can be "in the ballpark" when you're close or within a certain range: "I'm just guessing how much that car costs, but I bet I'm in the ballpark."