This is the part of your trucks A/C box that holds the internal flapper door which directs air flow to either the dash vents, floor vents or defrost vents. The diverter door is usually bad and does not seal properly letting air leak through the system. This leaking reduces air volume in all vents and affects the overall efficiency of the entire system. Often allowing heat to come in on hot days and allowing cool air from the A/C escape making it difficult to cool the cab.