you use a b&w image for the displacement map. The back areas would make a valley and all that is white would be the tip of moutains; all the gray is somewhere in between.
So if you turn that image that you have to grayscale, and then assign the file to a materials displacement, and then assign the material to the plane, when you render the pane it will look as if you had modeled the mountains and sutff.
A displacement map is "like" a bump map, except that it actually changes the geometry (so you use to for "Bigger" bumps than just bump maping.