A belfry is a part of a tall tower or steeple that holds bells. Those bells are loud — you would not want to be up in the belfry when they start ringing at noon!
A typical belfry is at the top of a church steeple. It's open on all sides so the sound of the ringing bells can escape — if you look up, you can see the large bells through the openings in the belfry. The word comes from roots meaning "to protect" and "peace," and the original meaning was "wooden watch tower." These ancient belfries held watchmen, but also warning bells. The phrase "bats in the belfry" describes someone who's quite eccentric.