A mobile phone device or a camcorder, when recording a video, records the visuals and the sound in one place.
But when movies are filmed, the visuals (by the camera) and the sound (by the audio recorder) are recorded separately, and then put together during the editing phase and carefully matched.
A clapperboard has two parts: the clicker (the top part that taps) and the slate (the bottom part on which the clicker taps).
The slate has some information written on it, including the scene and the take number.
As soon as the audio recorder and the camera start recording, the clapperboard operator places the clapperboard in front of the camera so the camera can see it, reads the information about the scene and take number so the audio recorder can hear it, and then claps the clapper.
When the editor starts the editing, he picks the sound of the click on the audio and matches it with the visual of the operator clapping.
This makes it easy for the editor to synchronize the moving picture with the sound.
