A Plenum is a large area of space that holds the intake charge in a manifold. It evenly distributes the air to each cylinder in the engine (or cylinder bank on a naturally aspirated car). The size of the plenum allows each cylinder to draw an equal amount of air and provides less turbulence when opening the throttle. The plenum also has ports for the fuel injectors on top of it so that fuel is injected just before the air enters the cylinder head.
The plenum is used for many flow applications such as wind tunnels, rockets, and automotive intakes to even out pressure differences between cylinders and increase overall volumetric efficiency. In an automotive application it works by generating vortex that create cycles of oscillation which equalizes the pressure of the air when entering and exiting the system.
When the plenum on your Audi becomes clogged, it can affect your car's ability to breathe, leading to reduced power output and other problems. In order to avoid this, it's important to keep up with routine inspections and maintenance.
The plenum tray is usually located underneath the front windshield of your Audi. It serves several purposes including water drainage, aesthetic cover to hide the gap between windshield and bonnet, hiding wiper motors and mechanisms etc. It's important to ensure that the plenum tray is always clear of dirt and debris to prevent water leakage which can damage the HVAC blower and other components within the cabin.