iCloud: two-step verification
Two-step verification is an optional way to increase the security of the Apple ID you use to sign in to iCloud. It requires you to verify your identity using one of your devices or your Recovery Key in addition to entering your password. Here’s how it works:
You enter your Apple ID and password as usual.
Apple sends a temporary verification code to one of your devices.
You enter the code to verify your identity and finish signing in.
Note: Two-step verification isn’t an option if two-factor authentication is available for your Apple ID. (See the Apple Support article Availability of two-factor authentication for Apple ID.)
You can set up two-step verification on the Apple ID account website. When you set it up, you can register one or more trusted devices that can receive verification codes using either Find My iPhone notifications or SMS. You also receive a Recovery Key to use if you don’t have access to a trusted device.
See the Apple Support article Two-step verification for Apple ID.