Hidden Pages are pages that are not displayed to participants. Hidden pages do not show on the menu. You can hide them by dragging them below the line at the bottom of the pages table in the Admin Panel. See the screenshot below for further clarification:
The Survey page is dragged below the line, so it will not be displayed to the participants.
You can create pages as you like and choose when to show them or hide them using the Admin Panel.
Note that hiding a page will not change its contents and it can be used as it is when it is shown on the menu.