When uploading a medium or editing it in the backend you can set different publish states: public, hidden or private.
If the option "hidden" is not available, please enable "Hidden media direct link" within the backend at Configuration -> Media.
Frontend (media upload):
These options define, if all users (public) or only the owner/upload (private) may access the medium.
Special option "hidden":
If you select the option "hidden", a medium can only be accessed via a special URL (the direct link) that can be retrieved by the medium owner only within the share dialogue in order to pass it to special recipients who shall be able to view it. The URL contains a long token in order to secure unauthorized access. "Hidden" media will not appear in the media lists.
Another special feature:
If a "hidden" medium will be assigned to a group or a channel, it will be visible within the group or channel context and thus can be made available to a defined group of users only.