To set a password or put in IP that allows watching a website, go to Site Settings → Privacy, put in login and password or IP.
Limited access is necessary if you want to restrict website visible for specific people, and if you want to forbid indexing a website by search engines, for instance, while you don't assign a working domain name or don't finish working on a website.
Also here you can find a search engine indexation restriction option. You can turn it on to forbid your site indexation in the robots.txt file.
How to set a password for a single page
Go to the Page Settings → Additional to set a page password.