A Smart Key is a single key that provides access to all of your wallets. The Smart Key is always held by the user and is protected by a backup key that can be written down and stored as a paper wallet. Ethos cannot authorize or spend any of your funds without your Smart Key. Your Smart Key is tied to your password and your backup key so that you always have secure access to your funds wherever you go.

You can find more information on Smart Keys here and here.