The Authentication key is a 16-character key that can be used to set up 2FA in the same way as scanning a QR code.
It can be saved and used as a backup to be entered manually in an authenticator app like Google Authenticator or Authy in case you lose access to the app (for example a lost or broken phone).
We advise you not to store authentication keys (or any other credentials) online.
Please note that this key can not be used as an authentication code.
Read more about 2-Factor Authentication here.