On a normal WordPress site, once a new user registers, the user is created in the database. Then an email is sent to the new user with their login credentials. Very simple. As it should be.
The New User Approve plugin modifies the registration process. When a user registers for the site, the user gets created and then an email gets sent to the administrators of the site. An administrator then is expected to either approve or deny the registration request.
An email is then sent to the user indicating whether they were approved or denied. If the user has been approved, the email will include the login credentials. Until a user is approved, the user will not be able to login to the site.