What was that "crack" thing when shooting?

That's a storyboard. Generally, there will be multiple storyboards on the spot. Before each scene starts shooting, the storyboard will indicate the camera, scene, actors and so on. Then put the sign in front of the camera, quickly read the information on the sign, then put the sign, and you can start shooting.

The purpose of the clipboard is to facilitate post-editing, because a movie, no matter how big or small, has to be made into hundreds. If there is no systematic record in the early stage, it is almost impossible to rearrange it in the later stage.

As for the sound of "card", it is to align the picture with the sound in the later editing.