This is a picture of a VU meter, it is from a late-1970s vintage Aiwa model AD 6700 cassette player | recorder.
Back before micro-chips powered numerical values on VFDs or LCDs the time left on tapes was usually displayed with a mechanical rotary clock like the odometer on a car dashboard.
The designers of the machine decided to simplify the user experience by providing an easy scale to read off. It was an elegant solution, particularly when slide rules were in common usage. So how to read it would have been self-evident, just as the window, mouse pointer interface of the first web browsers were familar to PC users.