A histogram often looks like the solid shape of a mountain range with a jagged top, but it is actually made up of a series of adjacent vertical bars that measure the amounts of each brightness level in an image. The brightness range extends from black on the far left of the graph to white on the far right. The taller the bar in the graph, the more of its tone is present in the image.
For example, an image of mostly white clouds generates many tall bars on the right side of the graph, representing the light colors, possibly some short bars on the left side of the graph, representing a few darker tones, and it might not have any bars on the far left side if there are no very dark blacks in the image. Add some black birds in the sky, however, and there would be a bar on the far left side of the image's updated histogram.
Figure 9.1 shows a light (high-key) image, a dark (low-key) image, and each image's histogram.
Figure 9.2 shows the histogram of the lighter image when black birds are added. The relatively few additional bars on the right represent the in-between values of pixels that blend the outer edges of the birds with the sky, which would be viewable by zooming in.