Single sign-on (SSO) allows a user to authenticate once to access multiple applications and resources. SSO provides a centralized identity management system, enables IT control over access, and provides a seamless experience for users between on-premise and cloud applications. SSO works by having users authenticate at an identity provider, which then issues claims to applications like SharePoint, allowing users to access multiple resources with one login without needing separate credentials for each system. Proper implementation of SSO in SharePoint requires consideration of additional factors like converting NTLM to claims, managing the people picker, handling sliding sessions, and enabling single sign-out.