That strange ring-shaped plastic tool is most likely a water-filter housing wrench, used to loosen stubborn filter canisters when replacing cartridges.
It fits around the filter housing and provides extra grip and leverage to unscrew it without damaging the unit.
Because these wrenches vary by brand and size, check any molded markings and make sure it fits your filter properly.
Before changing a cartridge, turn off the water and release the pressure, then keep the wrench near the filter for future maintenance.
