This is a 10 section step wedge to allow you to check that you have a full chromatic scale available on your monitor.
You should be able and see the 10 separate tones going from an all white to a total black.


Step back from your monitor.
One of these numbered boxes is about the same shade as the surrounding.

That's your "screen gamma."
Most photographer's web pages are designed for 1.8 Gamma.
TV monitors are calibrated for 2.2 Gamma.