Two factor authentication is a security system that requires two levels of trust - something that you know (i.e. a username + password) and something that you possess (i.e. a hardware fob/token or a smartphone).
The traditional approach to two factor authentication uses hardware tokens - a device that generates a one-time password (OTP) - to provide the second factor.
Without the code from the token, no one can login as you. While this approach works, it's cumbersome because you need to have the hardware token with you to be able to login to your IT systems.
Duo Security decided to replace hardware tokens with software based tokens that work on a device most people already have - a smartphone. Unlike with hardware tokens, you don't have to mess around with OTP passcodes in order to login.
Instead, the act of logging in with your username and password triggers an automated notification to get pushed out to your smartphone. To complete the login process, you open the notification on your smartphone and then tap on the Approve button.
Visit this link to see a video demo of Duo Push in action