OpenID is a decentralized protocol that allows users to log in to multiple websites using a single digital identity. It allows users to maintain a single username and password that can be used to access any website that accepts OpenID logins. When a user logs in to a website using OpenID, they are redirected to their OpenID provider to authenticate, and then sent back to the website. This allows for single sign-on across multiple sites without needing separate credentials for each site.