SSH authorized keys are public keys listed in a user’s ~/.ssh/authorized_keys file. When a matching private key is presented, SSH can allow login without a password. This is a common, secure way to manage access when keys are protected and the list is kept current.
Attackers with access to an account may add their own public key to the file, creating a way to return without reusing a stolen password. Defenders should review authorized-key files for unfamiliar entries, check file ownership and permissions, and remove keys that are no longer needed. Unexpected changes can indicate persistence, but should be investigated alongside login logs and other host activity.