Side walls are 2D walls that extend from the height of the surface down to the Z axis:
The Side Walls tab, located in the Plot Details dialog, provides controls for displaying the side walls and customizing their color.
Select this check box to display X and Y side walls in the graph.
Select a color from the drop-down list for X and Y side walls. To hide either the X or Y side walls, select None from the associated drop-down list.
To set the transparency of the side walls move the slider or enter a desired integer from 0 to 100 in the combination box. Note that 0 means the symbol is not transparent at all, and 100 means the symbol is fully transparent.
Check the Auto box to using the transparency setting in surface tab.