An HVAC blend door actuator is a small electric motor connected to a door mounted inside the air ducts of the HVAC system in your vehicle.
When you set the temperature level on the control panel, the HVAC electronic control module controls a small door inside the ducts to redirect air through or around the heater core. When you turn up the heat, air passes through the heater core and warms up. When you turn the temperature all the way down, the air is routed around the heater core so that no heat is picked up. A setting anywhere in between results in a range of blend ratios. The device that responds to the control module to move the blend door is the HVAC blend door actuator.