An image histogram is a type of histogram that acts as a graphical representation of the tonal distribution in the image. It plots the number of pixels for each tonal value. By looking at the histogram, a viewer will be able to judge the entire tonal distribution at a glance.
The horizontal axis of the graph represents the tonal variations, while the vertical axis represents the number of pixels in that particular tone. The left side of the horizontal axis represents the dark areas, the middle represents neutral one and the right hand side represents light areas. The vertical axis represents the size of the area that is captured in each one of these zones.