That's the flapper door for the HVAC system. If you run the HVAC system you can see it open and close. When it's open, it's recirculating and when closed it pulls all air from outside. I've learned that this door does not simply open/closed based on the recirculating button on the dash. If you have the HVAC system set to auto, it does not matter if you have recirculating button pushed or not, the system will open/close that door as it sees fit....it overrides your button on the dash. Also, I'm about 90% sure that the humidity sensor controls will also override that recirculating button on the dash if the cab gets too humid inside and it thinks the windows are in a "fogging" condition. Personally, I think the recirculating button on the dash should override anything else, but it does not work that way....annoying.
