A bang is a loud noise, like a door slamming or something heavy being dropped on a wood floor. Hearing a bang outside your house at night might make your dog start barking.
The sharp sound of a bang might be made by someone pounding on your front door, or two cars colliding. A bang can also be a painful physical knock, like a bang on your head when you crash your bike. You can use it as a verb as well: "Don't bang your laptop down on the table — you'll break it!" Bang shares a Scandinavian root with the Old Norse banga, "to pound."