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.
My heater and ac fan only works on high when i turn my switch on for the fan on the heater and ac it only works on full blast.
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 the resistor block tests out good and you don t have any fan speed or a slow fan speed for all speeds you can assume that the blower motor is at fault since everything else has tested out ok.