Since the video camera is often at a distance from the recorded scene, the audio recorded by the camera is of lesser quality (less loud, with interferences for other audio sources, …). To compensate this loss you have to put a microphone near the recorded audio source, using a better quality microphone and an audio recorder dedicated for this task (with hardware recording audio with a higher quality). Consequently you have audio recording separate from video recording, and you need to synchronize the two recordings afterwards.

A simple trick to help with this is to have a clap, for example just by clapping hands. This produces a sharp short sound easily identifiable on the audio track, and on the video recording the clap is also visible. You can then synchronise the two recordings, with a precision of up to a frame. Furthermore the video camera often also records audio, and since the clap is loud it is also present in this audio track, allowing synchronising the audio tracks themselves with a higher precision than frame-based (although it will not be “visible” in the resulting mix).
I actually even wrote a script which does this automatically, provided the two corresponding recordings.

The other difficulty is that you need to keep track of which audio recording corresponds to which video recording. This is where the clapperboard comes into play. There you can write which take you are currently recording (and other information such as scene, date, …) which is visible on the video recording, and have someone announce it so to be also present on the audio recording.

And instead of having to erase and rewrite the take number after each take on the board I wanted to have a digital display, and just change the number using buttons. The same way I could show the scene, the episode (for my podcast), and even the date and time. But also because the video and audio recordings use numbers in their file name, the displays will also show these. This allows me to know which audio file corresponds to the current video file, without having to cross reference the take numbers in all video and audio recordings. And instead of having to announce the take number so to be present in the audio recording, a buzzer with take care of this task instead of myself.