A return air grille is a connection to duct work that allows air to return to a heating and cooling system.  Return air openings are typically covered with grill work that serves a number of different functions.  The return air grille covers a register that allows air to flow into the heating and cooling system.  Most grills are adjustable, allowing people to shut a room off so the furnace cannot pull air out of it.  The amount of air allowed through the air grille can also be moderated, as for example if people want more cold air pulled out of downstairs rooms in the winter and less of the hot air upstairs pulled through the heater.  Made of Galvanized Iron, aluminum or stainless.  Available in square or round versions.