Single sign-on (SSO) is an authentication method enabling users to access multiple applications with one set of login credentials by establishing trust between a service provider and an identity provider, such as FusionAuth. This trust is built through cryptographic credentials, allowing seamless authentication and reducing password fatigue and IT support tickets. Security is enhanced through additional features like multi-factor authentication without adding complexity to applications. The tutorial demonstrates implementing SSO using Next.js, FusionAuth, and NextAuth.js, highlighting the setup of FusionAuth, application configuration, and user registration. It guides creating a Next.js application, either by cloning an existing demo or building from scratch, with steps for integrating and testing authentication functionality. Configuration involves setting environment variables, and the application is tested by starting a server, logging in using FusionAuth, and verifying user session handling within the app. Future enhancements could involve customizing FusionAuth's appearance, exploring OAuth, and developing a protected page in Next.js.