Two-factor authentication is a way of proving your identity based on your username and password as well as a physical device that you can carry with you. This makes it much harder for crooks to steal your identity, since they will need access to your phone or hardware token - not just your login credentials.

They've chosen the free service offered by Duo Security that lets you approve the device in several ways including a phone call, SMS tokens and push-based authentication. They walk you through the setup of a Duo account, making an "Integration", grabbing the PHP and Javascript files needed to make it work and changing up your user and administration module to send a bcrypted value to the service. They flesh it out with a new version of the view to include the Duo code and generating the signed request.