Two factor authentication is a security measure that requires users to provide two forms of verification to access their accounts. In this article, Phil Nash demonstrates how to implement two-factor authentication using Authy in a Rails 4 application with Devise. The author starts by setting up a starter application and installing the necessary gems, including Devise and Devise-Authy. He then creates an initializer for Authy, sets up API keys, and runs install scripts to integrate the service into the application. With two-factor authentication enabled, users must provide a second form of verification, such as a mobile phone number or push notification, in addition to their password. The author concludes that two factor authentication is a simple way to improve security, requiring only a few lines of code and minimal setup.