What is a black rectangle when shooting a movie?

It's not necessarily black. There are wooden boards and electronic boards with movie names written on them. Which scenes, which shots, which are easy to find, because the movies are not made in the order you see them.

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Generally speaking, when all the equipment such as cameras and tape recorders are running, the actors are ready, and when the director calls start, it means the official start.

The recording and the picture are separated, but the camera recording or tape recorder can't see the picture. In order to combine the picture and recording later, it is necessary to have an identifiable point. This is the moment when the boards on the picture click together, which can be found in both the picture and the recording.

I wonder if this explanation is clear.