Jean Grey herself is an Omega-Level mutant with powerful telepathy and telekinesis. The Phoenix Force is a cosmic entity described as the source of both the creation and destruction of the Multiverse. It has a symbiotic relationship with certain powerful telepaths, but Jean Grey is considered the pinnacle of its physical incarnation.
https://marvel.com/universe/Phoenix_Force
Its power is limitless and it doesn't really do it justice to describe it as "altering reality at will".