An actuator is a device that allows the flow of air into your vehicle after either the air conditioning system (A/C) or heating system has been activated. You can find yours by entering your registration here.
The many components of a vehicle's A/C or heating systems combine to pump air through to the interior of your car, but none of it would come to fruition if it wasn't for a fully functional actuator.
Actuators are effectively electrical motors, but they are not too fiddly to replace. Removing an old one and replacing it with one of slightly different dimensions might mean that a recalibration is in order. If this is the case, it is advisable to consult a mechanic.