Gaming boards
Gaming boards are actually the cover for the table and represent the ground, air, or sky, depending what period or type of wargame you're playing.
We mainly use felt of various colors to properly represent the ground. Green is good fo grass and fields, and tan is good for deserts.
More elaborate types can be made bycovering sheets of of styrofoam (expanded polystyrene to British or those white densely packed bead boards to everyone else) with filler and then spray it the desired color and cover with grass/snow material from modelling shops/sand.