A damper (sometimes called an adjustable louver) is an adjustable mechanism that sits behind the grille inside the duct opening. It lets you redirect airflow or close a vent partially or fully — useful for balancing temperatures across rooms or shutting a vent in an unused space. 

Louvers are sold separately from most of our grilles and are installed inside the duct before the grille is placed. New HVAC systems typically manage airflow balance at the unit itself, so louvers aren't always necessary — but they remain a popular option in older homes or for anyone who wants room-by-room control. 

You can partially reduce airflow using a register with a damper, but completely closing vents is generally not recommended by HVAC professionals — it increases system pressure and can reduce efficiency or cause damage over time. 

If you want to limit air to a room, the best approach is a register with a damper set to a low (not fully closed) position or consulting an HVAC technician about properly balancing your system.