Plenum is the enclosed space inside a structure that is used for airflow. It is often referred to as the space between the structural ceiling and a dropped ceiling or a raised floor and the hard floor in a building. Plenum is part of the structure itself. This air-distribution box is attached directly to the HVAC supply outlet which either cools or heats a home to make it comfortable.
The ductwork that distributes this air to the individual rooms of a structure connects directly to the plenum. The HVAC equipment themselves have a blower that forces the heated or cooled air into this supply plenum and from the plenum itself into rooms through the supply ducts.
In forced-air systems, there are also return ducts that connect directly to the HVAC equipment itself through the return plenum. The return plenum is connected to the air-collection box. When the cooled or heated air is blown into a room the other air is sucked into the return ducts and gathers within the return plenum.
A professional HVAC professional can evaluate the workings of an existing HVAC system to determine it is working to its optimal capability.