The key allows Chibiusa to travel through various timelines, and call on the Sailor Soldier of Space-Time, Sailor Pluto herself. Sailor Pluto also wears more of these keys on a Chain around her waist.
Guardian of time! Tear apart the sky and open the Door of Space-time to me! I call your true name, the almighty god of time, the guardian of time's father, Chronos! Guidance to me! Protection to me! The path of light to me!