This feature allows you to restrict the user login based on configured domains. You can allow/deny the user login based on email domain.
You can find this setting in the Sign In Settings tab -> Advanced Settings -> Restricted Domains
You can add the domain in the configuration, like shown below, separated by commas (,)
If you enable the Allow restricted Domains option then, users will not be able to log in using the configured domains in the restricted domains field.
When the user tries to login from a restricted domain, he will be denied the login and shown this screen: