Have you noticed the graph that pops up when reviewing your photos on your camera and wondered what it is?  It is simply a graph that displays the range of tones present in that image. The far left side of the graph represents black and the far right side of the graph represents white, with gray tones in the middle.  Between the two are all of the intermediate tones between black and white.  The height of the bars indicate how much of that shade of light the photo contains. Photos that have a lot of dark tones or are under exposed will have more bars toward the left side of the histogram and photos with a lot of white tones are that are underexposed will have more bars toward the right side.  Check out this article on image histograms with a lot of examples of photos and their corresponding graph.