A clapperboard consists of two pieces of wood that are connected by a hinge and hit together before each scene when making a film, to make it easier to match the sound and pictures of different scenes.
[British]regional note: in AM, use clapboard
clapperboard in British English
(ˈklæpəˌbɔːd)
noun
a pair of boards clapped together during film shooting in order to aid sound synchronization