Single sign-on (SSO) is a centralized user management system that solves the issue of multiple inter-related applications requiring separate user management systems, resulting in redundant data and increased complexity. SSO eliminates the need for users to remember different credentials across applications, providing a seamless authentication experience. FusionAuth is an identity provider that acts as the central hub for user information, communicating with multiple applications through secure protocols like SAML and OAuth. The process of integrating SSO into a Drupal site using FusionAuth involves installing the necessary module, configuring FusionAuth, and setting up the OpenID Connect client in Drupal. With this setup, users can log in to the application using their email and password, or register for an account directly through the application, eliminating the need for separate user management systems.