Omniauth gem allows easy integration of over 60 authentication providers like Google, Twitter, GitHub, and Facebook into a Rails application. The document outlines the steps to add authentication via Facebook to a Rails app using Omniauth, which includes signing up as a Facebook developer, adding provider credentials to the Omniauth initializer, generating a Sessions controller, and setting authentication routes. Once configured, users can log in with their Facebook credentials and an auth hash containing profile data will be returned.