For cameras a histogram is a graphic representation of the tonal distribution. It can tell you where all of the brightness levels contained in the scene are found, from the darkest to the brightest. The horizontal axis represents the tonal variations the camera is capable of recording. The vertical axis represents the amount or size of the area which is captured in each one of these zones of tonal variation.
Its an oversimplification but the shadows and dark tones show on the left, bright most lit on the right and midtones between. You can do histograms for the whole image or break them into channels usuall RGB. If you learn to read a histogram it can tell you where you have clipped details in the shadows and blacks or in the bright white areas.This would be shown with the high areas or the graph up against either the right or left wall of the histogram. It can tell you if your exposure is alright, In a low key image the histogram shows that almost all of the data in the image is down in the lowest areas (darkest) to the left. In a high key image just the opposite happens . Almost every value seen is toward the right side of the histogram.
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