It's oneness with everything, the Goddess; reality, not any particular cosmology.
You as your brain lives for 5-15 minutes after your death ingest an endogenous chemical dream cocktail that makes eons of seconds, makes one more aware and brings one close to the universal creator and exister, Love, the Abyss, Space, Space-time, Matter-Energy, the Mother of the Dao in tune with the Dao, Sunya(ta), the Underlying Pirnciple, Prime Mover, Nammu, Nuit, etc. You realize all the sacred truths and live lives accordingly.
Now you need to know a bit of cutting edge science. Consciousness is composed of quantum stuff. SO when your body recycles, as all things do, you go back into the Well of Souls, The Abyss, and become one with her. It's like you become Nothingness. You cease your normal existence and merge with the universal consciousness.