The password page is the page your customers see when your store is password protected.  Merchants often enable the password page while they are setting up their stores and don't want customers to see their products and pages yet.  Here is a great article from Shopify on how to enable your password page, how to customize your messaging, and how to access the theme settings.  Read all about it here:

Our ShowTime and Galleria themes allow you to add a custom background image, adjust some text options, add social media icons, and provide a signup to your newsletter.