When you log in to your account you have to provide your password and also use the key. You can choose a key that has an additional factor built in, such as a

When you log in to your account you have to provide your password and also use the key. You can choose a key that has an additional factor built in, such as a fingerprint reader. If you lose one of the keys you can authorize a new one. If you lose both of the keys and all the Apple devices that you can log into directly, e.g. with fingerprint sensor on a Mac, then you're SOL.