1987 Chevy Silverado resister

The resistor is the most common electronic component and is used to limit and/or control the voltage and current in an electronic circuit. I'm assuming you are talking about the blower motor resistor. In this application, the blower motor resistor will limit the currents going through each fan speeds wiring. So when you select speed 1, the resistor will not allow enough current to go through to allow speed 4 to work etc.