When you first set up 2FA, you're provided with a 24-digit recovery key. You're asked to note this down and keep somewhere safe.
This code allows you to log in when you're unable to authenticate using your primary 2FA method (Authenticator app, text, or call).
The recovery key is a single-use code. After you use it, you receive a new recovery key to use next time when needed.
You can generate a new recovery key if you don't know it or have lost it.
If you've lost your recovery key and need it because you can't authenticate, use your backup method instead.
If you don't have your primary authentication method or the recovery key, use your backup method to log in.
Keep your recovery key safe, so that you can log in using it in the future if you're ever unable to authenticate.