What it means
A spare security key is a separate physical authenticator registered to the same account in advance. It can provide a usable sign-in or second-step route if your daily key is lost. It is not a copied key and its presence in a drawer helps only if the account already recognizes it.
A real-world example
Two keys travel on the same keyring and the whole ring is stolen. That is not resilient. One key at home and one carried, plus another recovery route, gives a better failure separation.
What to do
Register both keys, label them in account settings and perform a sign-in test. Remove a lost key's entry promptly after regaining access through the other route.
The distinction that matters
A newly added Google key may not be usable immediately; Google documents a possible trust delay. Security keys used as second factors and FIDO2 passkeys on keys also have different sign-in experiences.
Google's security-key help
