An Authentication Protocol is a set of rules that determines how users prove their identity to a system. It ensures that only authorized individuals can access sensitive information or resources. Common methods include passwords, biometrics, and security tokens, which help verify that a user is who they claim to be. These protocols are essential in various applications, such as online banking and social media, to protect user data. By using protocols like OAuth or Kerberos, systems can securely manage user identities and prevent unauthorized access, enhancing overall security and trust in digital interactions.