A User Principal Name (UPN) is a unique identifier for a user in a network, typically used in environments like Microsoft Active Directory. It resembles an email address and is formatted as "username@domain.com". The UPN helps in identifying users across different systems and services, making it easier for them to log in. The UPN is important for single sign-on (SSO) solutions, allowing users to access multiple applications with one set of credentials. This simplifies the user experience and enhances security by reducing the number of passwords that need to be managed.