The tutorial explains how to set up a basic Ruby on Rails application to securely authenticate with an OAuth2 server using the authorization code grant. It uses FusionAuth, a customer identity and access management platform, as the authentication provider. The tutorial covers setting up FusionAuth, creating a new application, registering a user, and implementing the OAuth flow in the Rails application. The implementation includes handling the login process, exchanging the authorization code for an access token, and verifying the claims on the received access token. The tutorial concludes with polishing up the Rails session and provides suggestions for next steps, such as adding user roles and customizing the FusionAuth login page.