The plenum is the heart of a forced-air heating, ventilation and air conditioning duct system. The plenum connects the duct work to the mechanism that generates the heat or cold air.
Basically, a plenum is an air-distribution box through which air is forced. There is a supply plenum and a return plenum. The supply plenum is fed air from a blower that is then routed through ducts and forced into the various rooms of a home. The return plenum collects used air that is drawn in from the home through return ducts for another cycle of heating, cooling and distribution.