Firebase Authentication is the sign-in service of Firebase, Google's platform for building mobile and web apps. Authentication means checking who a user is before an app shows or saves that user's data. Firebase provides the backend service, SDKs (code libraries for iOS, Android, web, Flutter, C++, Unity and servers) and ready-made sign-in screens.
The app collects the user's credentials, such as an email address and password or an OAuth token from an identity provider like Google, Apple, Facebook or GitHub, and passes them to the SDK. Firebase's servers verify them; the app then knows the user's profile, can limit access to data in other Firebase products, and can send the user's token to its own backend as proof of identity. Phone-number and anonymous sign-in are also supported, and FirebaseUI is an open-source component that draws the sign-in screens. The upgrade to Firebase Authentication with Identity Platform adds multi-factor authentication, SAML and OpenID Connect sign-in for company identity systems, audit logs and multi-tenancy.
It is a hosted Google service: free up to a monthly number of active users, then billed at Google Cloud prices; phone sign-in is billed per SMS.