A plenum chamber is like a room where air is stored before it goes into a heating or cooling system. Imagine you have a big box where you keep all your toys. Before you take a toy out to play with it, you reach into the box and grab it.
The box is like a plenum chamber – it stores air before it gets sent to the heating or cooling system. When the system needs air, it can reach into the plenum chamber and grab some. This makes sure there is always enough air available to keep your house nice and warm or cool.