A blower motor resistor is an electrical component that is part of the heating, ventilation, and air conditioning (HVAC) system in your vehicle.
Housed in a special duct below your dashboard ahead of the glove box is a fan - the blower motor. The blower motor is responsible for introducing either warm or cool air into the passenger compartment when you turn on the heat or AC.
In order for you to be able to adjust the speed of the fan, a resistor is inserted into the electrical pathway leading to the blower motor. This blower motor resistor alters the amount of electrical current flowing to the fan motor in response to changes you make to the climate controls.