Oneness was a state of union with the Force. Usually, when a life-form died, they would become "one with the Force" by becoming a part of the Cosmic Force, which then provided power to the Living Force.
During the Battle of Exegol, Rey achieved the state of oneness when she called on to the spirits of all the departed Jedi. The voices included her former Jedi Master Luke Skywalker, his father Anakin Skywalker, Obi-Wan Kenobi, Yoda, Qui-Gon Jinn, Ahsoka Tano, Mace Windu, Luminara Unduli, Aayla Secura, Kanan Jarrus, and Adi Gallia, who encouraged her and lent her the strength to carry on against the Dark Lord of the Sith Darth Sidious.