During her confrontation with her paternal grandfather, Darth Sidious, Rey was able to achieve a state of oneness thanks to the aid of the spirits of all Jedi who had passed on.
Oneness signifies a unified state with the Force. Typically, upon death, beings transition into a state of "oneness with the Force," integrating into the Cosmic Force, which subsequently empowers the Living Force.
In the midst of the Battle of Exegol, Rey attained this unified state by summoning the spirits of all Jedi who had become one with the Force. Among the voices were those of her former Jedi Master Luke Skywalker, as well as his father Anakin Skywalker, Obi-Wan Kenobi, Yoda, Qui-Gon Jinn, Ahsoka Tano, Mace Windu, Luminara Unduli, Aayla Secura, Kanan Jarrus, and Adi Gallia. They offered their encouragement and bestowed upon her the strength necessary to persevere against the Dark Lord of the Sith, Darth Sidious.