A "ballpark" is a rough estimate. In other words, it's a guess about an amount that's close to the correct number, but not exact. You use it like this:

Here's an estimate of the cost. Keep in mind, it's just a ballpark.

We'll call up the electrician and have them give us a ballpark, but I'll check with you before I have them do any work.

This phrase comes from the sport of baseball. A "ball park" in a place where people watch baseball games. If two locations are within the same ball park, then they're not exactly close to each other, but they're not too far away either.