The blend door actuator is a servo responsible for controlling the mix of warm air from the heater core and cool air from the A/C and outside. If it malfunctions, your climate control system may not respond properly when you change the temperature on the climate control system.
Both the manual and the automatic systems use this servo to control the blend door. If your ATC system fails to automatically change temp, this is also an indicator that the servo has failed.
See this repair guide for the air mix mode blend door actuator.