For the purpose of closed testing, or limiting access to a project, adding a password is an easy way to control who can view your page. The feature is part of the access control options that make up refinery. You can set this option from the bottom of your project's edit page, by setting your page to restricted.
You can provide a password that must be entered by anyone who wants to visit the page. People who already have access for another reason do not need to enter a password. You can also include the password directly in the URL so it's not required to type it in.
Password protecting your page not only allows for closed testing, it also permits a content creator to permit specific people access to offer feedback before a page goes public. It's a great way to ensure that you and your team are satisfied with the content and appearance of the page.