A slate (AKA a clapboard, clapper, sticks, sound marker, and probably a dozen other names) is the tool that most people think of the most when you mention film production. But what is it for?
A clapboard is a tool that has two uses. The first use is to provide scene information. There are spots on the front to mark the day, location, scene, take and several other bits of information. In postproduction, the editor can group them according to these elements and keep their workflow organized.
The second use is to assist in syncing sound. On professional sets, the sound is recorded separately, and is matched up later. When the person in charge of the slate claps it, there is a peak that shows up on the audio track. The editor will be able to line up this peak with the closed clapboard so sync sound more effectively.
Do you need to clap it when you’re not recording independent sound? No, but it’s fun and you’re probably going to do it anyway.