A histogram is that graph looking thingy Mike posted above his picture (above), most higher end digital cameras and imaging software, Photoshop, have them. It allows you to see where the shadows and highlights fall within an image. Ideally you would want all that "black mountain" looking area to be in the center between the left and right, not touching the edges. The left side of the histogram is the shadow area of your image, if the black goes all the way past the edge you have lost or blocked up shadow detail, if it goes beyond the right side you have blown highlights. All of this is also subjective to your particular image.
BTW, "black mountain area" is not the technical term, its just easier to explain that way, I didn't want to go techy on you.
Todd