In the above histogram the tonal range is equally spread across the whole range left to right, the shape of the histogram will dictate the tonal range and brightness/darkness of the photo. In this example the high peak around around the left hand side of the histogram means there are lots of dark tones in the photo, there are very little mid tones and there is quite a lot of bright tones.
This is good example of a high contrast scene. A low contrast scene would have a large peak in the centre of the histogram that would taper off to the shadows and highlights, essentially giving us a photo that comprised of a lot of grey tones. (read Correcting Exposure with the Shadows and Highlights Tool)
The above histogram shows the effect from over exposing the photo by 2 stops which has pushed a lot of image detail into the highlights and off the scale, straight line on the very right of the histogram shows us that the highlights have been clipped, these clipped highlights would be pure white with no detail or texture.
On the other hand if we under exposed the scene by two stops we would get a histogram like the one below.
In this case there are no clipped highlights however we have clipped or blocked up the shadows, this will give us the same problem as before with exception this time it will be shadows that are captured as pure black without and detail, again this is not ideal as we are not capturing the full tonal range of the scene in front of us.
Quite often when photographing in the landscape we are presented with a scene where the sky is much brighter than the ground especially during sunrise and sunset when there is little if any illumination on the ground, if we are aiming to photograph the scene without the use of filters or HDR imaging I would expose the maximum time before I started clipping highlights, there would inevitably be some clipped shadows but we would be capturing as much of the tonal range as possible.
When you have a tonal range that is far too big for the camera to record you are left with two options, you can use a graduated neutral density filter to balance the brighter sky against the foreground or you can use HDR imaging to record the whole scene, I will cover both of these topics in the near future.
Related Article Understanding a Digital Photography Histogram.