In Microsoft FrontPage, hidden folders can either be folders that site visitors cannot browse, or folders that are invisible to you when you're using FrontPage.
Folders that site visitors cannot browse
When you create a new web site, FrontPage creates a hidden folder named _private to store form results. Site visitors are not allowed to view the contents of this folder.
Note Depending on the Web server on which your web site is hosted, there may be other options for making sure site visitors cannot browse folders. Contact your Web server administrator for more information.
Folders that you can't see when using FrontPage
FrontPage also creates hidden folders that you can't see in the Folder List, but which can be browsed by site visitors. For example, if you use themes, FrontPage creates a hidden folder called _themes to store them.
In addition to being hidden from FrontPage views, hidden folders of this type are excluded from the FrontPage search component (if the search is being performed on a server running Microsoft FrontPage Server Extensions).
Note You can choose to show hidden folders on the Web Settings dialog box.