An important element of landscape photography is ensuring that all of the key elements within your composition are sharp. This can often include foreground objects that are a matter of meters from your camera as well as background elements that can be kilometres away. Therefore, to achieve this, you need to ensure your depth of field is large enough to render everything of interest, suitably sharp.
When you focus on an individual point within your landscape, you are in fact creating a plane of focus that lies parallel to the sensor. Everything in front of, and behind, that plane is technically not in focus, however there is a region within which objects will appear acceptably sharp – that is the depth of field. For instance, in the scene below the camera has been focussed on a rock. Therefore, the plane of focus lies parallel to the sensor across that rock and the limits of acceptable sharpness, that form the bounds of the depth of field, lie in front of, and behind, that plane.