Reman (or remanufactured) toner is a toner cartridge that uses the same original cartridge body but the toner powder and the microchip that talks to your printer is purchased from a third party.
In many cases that third party is the same that the OEM company is buying from but the company that remanufactures and sell those cartridge has to go through extensive testing to make sure that their product will function as expected.
One of the big players in this field is Xerox who is using their superb manufacturing process to remanufactures toner cartridges for other manufacturers, enabling Xerox to service printing fleets from all brands.