The Elder Scrolls Online servers use an additional layer of security beyond username and password. Anyone attempting to login to your ESO account from an unrecognized device must provide additional authorization in the form of a special access code that is sent to your contact email address. This code must be entered into the ESO client before your login is complete. The code is only valid for a total of 10 minutes from the moment unauthorized access is suspected.
This feature is enabled on all ESO accounts and it is not possible for clients to enable or disable it at this time.
Tthere is no detailed information about this service provided online at this time.