Yes it is the clapperboard, a black and white board with a hing at the top which allows the user to clap the top with the main board, making a sharp noise to anyone listening.
So what the heck is it used for? Originally the clapper was separate and specifically designed to sync the audio and video components of a film production. The sharp noise can be matched for both recordings, bringing the audio and video (which are often recorded by separate units) back together.
The board is used to record information about the scene for future reference. Originally the clapper and the board where separate, but an ingenious stage hand realized that both could be combined, making it a more functional stage tool.
Modern clapboards use LED’s and other technologically advanced information tools, and someday may emit a pulse noise that is digital rather than manual (the clapping of the boards). However, there is something nostalgically romantic about the clapping of the board that will likely remain in film for many years to come.