Landscaping is arguably one of the most important features in the editor. Landscaping will ensure your map looks natural, and give it a sense of height. Landscaping allows you to paint hills and clifs on your map, which can be used for various things.
Using the 'raise/lower' tool, you can raise or lower the current terrain level. Control and your left mouse button raises the land while control and your right mouse button lowers it. There appears to be no "minimum" height which I've discovered, and the same seems to be true about the the maximum height. But if you go down far enough, you'll get a pretty odd result:
Additionally you can alter the brush size, the brush type (so you can alter the area that is covered and how well) and you can also alter the brush integrity. This means how strong the brush will affect the ground.