A rough numerical estimate

"Do you know how many women Casanova slept with?" "I haven't the foggiest idea" "Gimme a ballpark figure"

"Sai con quante donne è andato a letto Casanova?" "Non ne ho la più pallida idea" "Dimmi un numero, più o meno"

"Can you give me a ballpark figure on the cost of my renovation project? "$ 80,000.00, give or take a few hundreds

"Può darmi un'idea approssimativa del costo della mia ristrutturazione? "$ 80.000, cento più cento meno"

The term refers to a baseball field. It's a development of the previous in the ballpark, meaning "within certain/reasonable limits", and out of the ballpark, meaning "beyond certain/reasonable limits"