How does that relate to our Histogram?

In the camera sensor the amount of light falling is measured in as a proportion of 255. Each pixel produces an output scaled 0-255
Black = 0
White = 255
The number of pixels at each level of brightness is therefore displayed on the vertical axis. In this image we have 1/3rd Black, 1/3rd Mid Grey, 1/3rd White.

In the camera sensor the amount of light falling is measured in as a proportion of 255. Each pixel produces an output scaled 0-255