Creating B&W images

[Ofnuts]

Each pixel has a "Value" (luminosity, more or less), somewhere between 0 and 255. The histogram shows, for each of these possible values of "Value", how many pixels have that particular value. In the histogram posted above, a large number of pixels have a value between 0 and 25, with a peak at V=12. Your normally want an histogram which is somewhat evenly stretched across all values. If it's too much on the left, the picture is underexposed, and too much on the right, overexposed. All decent digital cameras can display the histogram of the picture on the control screen. You can of course have specific histograms for the R,G, and B channels.

See https://en.wikipedia.org/wiki/Image_histogram