The return or return air duct is the duct that goes back (returns) to the HVAC system from the area that is being cooled or heated. A furnace or air conditioning unit must have a return on it so that it can cycle air out of the space it blows air into. When air is blown into a space, it increases the air pressure of the room so it has to also be sucked out and recycled through a filter on the HVAC unit. A return normally has a much larger grille than the other registers.
Cold air return on the wall:
