What is a hidden memory?
As a journal owner, it is possible to “hide” photos or videos during the initial upload. This keeps the moment visible only to you and to those followers who have been granted Full Access permissions (which makes them a co-owner of the journal with you). To those without Full Access, it would remain hidden.
Memories can be marked as hidden or "un-hidden" from the edit screen, but please note that this does not remove the memory from any automated email updates that have already been sent out to your followers.