Imported From: http://groups.google.com/group/in-portal-dev/browse_thread/thread/396e63ea1a9565f7#
2-factor authentication is when during a login user not only asked for his permanent password, but also a time-based 6 digit key which is generated on his "Google Authenticator" app. If you are using trusted computer, then you can check a checkbox, that would cause this 6 digit code not being asked for a month.
This way on a website if one of a users decides he needs a better security he can get it with this feature. Even WordPress has it: http://wordpress.org/extend/plugins/google-authenticator/