http://y7yea4pmqqtznb33qiugvysyn2bob5v62e4pvoadoibrwkq3tsddjeyd.onion/my/glossary/two-factor-authentication
Services that offer two-factor authentication also require you to provide a separate confirmation that you are who you say you are. The second factor could be a one-off secret code that is sent to you via email or text, a number generated by a program running on a mobile device, or a separate device, such as a USB authentication token that you carry and that you can use to confirm who you are.