Back in the stone age, you might remember, we had to record the events of our lives on video camcorders instead of our phones. These camcorders were great for amateur home use because they were pretty easy to use. Just point at something, hit a button and the audio and the video were recorded together in one place. This is different than how things are done in Hollywood, where the images and sounds for movies are recorded separately. The two parts have to be put back together in editing and carefully matched.
Why Do They Click That Board Thing Before Filming A Movie Scene?
A clapperboard is a device used in filmmaking and video production to assist in synchronizing of picture and sound, and to designate and mark the various scenes and takes as they are filmed and audio-recorded. Other names include clapper , clapboard , cue card , clacker , slate , slate board , slapperboard , sync slate , time slate , sticks , board , smart slate , dumb slate  and sound marker. When sound and picture are out of synchronization, there is a lip flap occurring. In the silent era the principal requirement of film stock identification during a day's shoot was the slate. Since the earliest sound films until the advent of digital cinematography visual and audio tracks were recorded on separate media by separate equipment.