Silver Key is a magical item.
The Silver 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.
O Guardian of time tear apart the heavens and open wide for me the door of space and time. I summon thee by thy true name, the almighty god of time father of the guardian, Chronos. Guide us on our journey protect these travellers grant us the path of light.