To password-protect specific pages or areas of your web site, you can use either the Password Protect Directories feature in your cPanel (just click on the Directory Privacy icon within the Files section), or if you are familiar with .htaccess Files you can use .htaccess to assign a password to a specific area of your site. Either of these methods will produce a login popup window similar to the one you see when you login to your cPanel.
If you want to protect a specific page, you must first create a special directory for it, put the page into the new folder, and then assign username and password access for that folder.