How does the board that gets snapped shut when "mark" is called work?

firestake

New member
Please excuse how horribly I've worded this question as I have no idea what the device is called!

In movies and TV, what is the name of the black and white board that gets snapped shut before a scene is shot? Also, why (these days) does it have digital numbers scrolling rapidly on it until it shuts? What purpose do they serve?

Thanking you!
 
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