] is a three-dimensional graphics directive which specifies that the front faces of polygons are to be drawn with the graphics primitive gf, and the back faces with gb.
The graphics specifications gf and gb must be CMYKColor, GrayLevel, Hue or RGBColor directives, or SurfaceColor objects.
Specifications given outside of FaceForm will apply both to the front and back faces of polygons.
The front face of a polygon is defined to be the one for which the corners as you specify them are in counterclockwise order (right-hand rule).
See the Mathematica book: Section 2.9.9.
See also: EdgeForm.
We load the package that contains definitions for polyhedra and show two graphics for comparison.
The FaceForm directive will have color the front (outside) of each polygon light gray and the back (inside) face dark gray.
Evaluate the cell to see the graphic.