If this box is ticked, users from the Internet will be able to register a new account with your instance of Curatr and gain access to the homepage. (Visiting the public Curatr registration page will show you an example of a registration form.)
This is desirable where you are looking to attract new registrations but is generally not required when organisations are using Curatr internally. In order to prevent public registrations, the box shown in the screenshot below should be unticked.
Just because your Curatr is set to ‘visible’, doesn’t automatically give users the right to view each course within your organisation. On a course-by-course basis, you can elect to allow users to self-enrol to the course, or to deny this privilege.