By looking at the histogram for a specific image a viewer will be able to judge the entire tonal distribution at a glance.
Photographers can use them as an aid to show the distribution of tones captured, and whether image detail has been lost to blown-out highlights or blacked-out shadows.
The vertical axis represents the size of the area (total number of pixels) that is captured in each one of these zones.
Conversely, the histogram for a very bright image with few dark areas and/or shadows will have most of its data points on the right side and center of the graph.
Because the information contained in the graph is a representation of pixel distribution as a function of tonal variation, image histograms can be analyzed for peaks and/or valleys.