A clapperboard

A clapperboard is a device used in film, television, and advertising production to assist with the process of synchronizing the sound and film recordings. Because sound and film are usually recorded separately, they must be matched carefully during the editing process, or the resulting production will feel slightly off to viewers. When the synchronization is extremely poor, it may become almost comic, but even small discrepancies can be very disorienting for viewers.
There are two components to a clapperboard: the clapper, and a slate. The clapper consists of two pieces of wood or plastic which can be snapped together to make a distinctive clacking noise which is easy to find on the sound recording. By matching the sound of the clack to the physical action on the film, the editor can synchronize the sound and film recordings. Clapperboards are sometimes also given away as mementos of film productions.
The slate is used to record information about the production, including the name of the production, the director, the director of photography, and the scene. The take number and camera angle are also written out on the slate. This information assists people in the editing room, allowing them to quickly know what they are looking at by reading the clapperboard.

Other names for the clapperboard include clapper, clapboard, slate, slate board, sync slate, time slate, sticks, board, and marker.