A histogram can be a very useful scope to judge the luma in your shots. It can also give you an idea of how well the shadows, midtones, and highlights of your image are distributed. The left hand side of the histogram represent pure black, and the right side represents pure white, and the greyscale move from darker to brighter as you move left to right. The height of the graph shows how much of your shot resides at particular luminance levels. Greg will show you a couple of examples so you can get a better idea of how it works.