What is a damper in ac terminology?

A damper is usually a piece of metal that is installed in a duct to allow the airflow to be restricted. This lets the user open or close the restriction to adjust the amount of air getting through. There is often a metal tab handle outside the duct to rotate the damper inside the duct. The handle is parallel to the restriction plate, so the least resistance is when the handle is parallel to the duct itself. To restrict the airflow, turn the handle so that it is up to 90 degrees off from the direction of the duct.