It means that when it only worked on high that the resistor in the blower fan wiring went bad and the only speed the fan would run on would be a speed where no resistor was being used which is.
Car heater fan only runs on high.
The fan in your car stopped working except when set to the highest setting.
Though having only one fan speed usually indicates a bad blower resistor a diligent mechanic at a repair shop will test the resistor fan switch and connections to the motor to make sure nothing.
If it doesn t light up or you only have a high fan speed then your resistor block needs replacement since it is bypassed on high speed.