ContourShading is an option for contour plots which specifies whether the regions between contour lines should be shaded.
False, regions between contour lines are left blank.
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, scaled so that it lies between 0 and 1.
See the Mathematica book: Section 1.9.6, Section 2.9.7.
This compares two contour plots, one with the contour shading on, the other with it turned off.
Evaluate the cells to compare the graphics.