An authentication provider is a configuration that allows users to log in to Salesforce using an external identity provider,such as Facebook, Google, or a custom one. When creating an authentication provider, two options that can be utilized are:

A custom registration handler, which is a class that implements the Auth.RegistrationHandler interface and defines how to create or update users in Salesforce based on the information from the external identity provider.

A custom error URL, which is a URL that users are redirected to when an error occurs during the authentication process. References: Authentication Providers, Create an Authentication Provider