A "hidden directory" is the given name to any folder that was made by a user, but will not be shown up on the dashboard. To make such a folder, a user must select the "New Folder" button, and type in any name that starts with a full stop ("."), for example, a user could make a folder called .music, once this is done, the folder will have been made, however, the only way the owner of a website can view the directory via the dashboard is to manually enter the URL of the folder, the URL should look something like this; https://neocities.org/dashboard?dir=%2F.(folder_name).
As you cannot normally view the folder via the dashboard, but it is impossible to delete the folder once it has been made.
All file types (HTML, PNG, MP3, etc) on such folders will be treated the same as they would on any other folder, however, the site owner will be unable to edit it via the online editor as it will keep on showing "Loading..." forever. Because of this issue, it means to update a file, a user cannot update it view the online editor, but rather by uploading it directly to the host (e.g. via dragging and dropping it).
A live example of such a directory can be found on the wiki, https://thewikion.neocities.org/.hidden/