A clapper board is a traditional filmmaking tool used to mark the start of a scene during film production. It consists of a hinged board with a slate that displays important information such as the scene number, take number, and production title. The clapper board is held in front of the camera and the slate is "clapped" shut, creating a distinctive sound and providing a visual cue for synchronization purposes when editing the footage. It helps ensure accurate labeling and syncing of audio and video recordings, making it an essential item on film sets.