What is an air medium in a heating system?

[Austin Youmans]

Got this message from my PM. Can anyone tell me what installing an air medium does?

"We received a maintenance request for the above property due to the heat not working properly. I dispatched a technician to inspect and diagnose. The charge for the diagnostic is $125.00.
They found that the return plenum is not properly attached therefore pulling dust and dirt in from the attic. Due to the age of the return, they recommend installing an air medium. This repair will be $750.00?