ContourShading is an option for contour plots which specifies whether the regions between contour lines should be shaded.
With ContourShading -> False, regions between contour lines are left blank.
With ContourShading -> True, regions are colored based on the setting for the option ColorFunction. The default is to color the regions with gray levels running from black to white with increasing height.
The value given as the argument for the ColorFunction function is the average of the values of the contour lines bounding a particular region. If ColorFunctionScaling -> True, it is scaled so as to lie between 0 and 1.
See The Mathematica Book: Section 1.9.6 and Section 2.9.7.