This is a histogram; it is a graphical representation of the tones in an image split into shadows, darks, lights and highlights. You can display this when you review your images by pressing DISP (Canon) or by pressing the down section of the control pad (Nikon) and it gives you idea of the amount of light and dark areas in an image.
The height of the graph in each sections shows how much of the area in your photo is taken up by that tone. The histogram above represents a photo which has lots of shadow and dark tones, but less lights and highlight tones.
If the graph goes over the left of the scale, it means that areas of the photo are underexposed and will these areas will show as pure black with no detail in the photo. A graph going off the right of the scale means that there are overexposed areas in the photo, which will be pure white and show no detail in the brightest parts of the image.
When your camera is in an auto exposure mode it will try to create a photo with an even spread of tones amongst shadows, darks, lights and highlights and the histogram should show an even distribution of the tones on the graph.