An authentication plug-in is a method of handling user authentication to Moodle when users log into your site.
You can have one or more methods as the same time enabled on your site, but each user can only use one method of authentication at a time.
You must enable and configure the method, as you need for your users in `Site administration` > `Plugins` > `Authentication` > `Manage authentication`.
Note that the order of processing on this page does matter and after `manual` and `nologin`, you should next put the method that most users will have.