Hidden SystemTags are those among them, that are visible only to you who used it, but not to anyone else who might take a look at your posts or tags.
With the last release, we introduced a new such SystemTag that does nothing:
sys:hidden:<somethingVerySecret>It does nothing? Why use it then? - Well, the feature simply allows you to use private tags. You may use any tag you like with the prefixes sys:hidden: or system:hidden: and no one else will see them.
The sys:hidden-Tag (as any hidden systemTag) is visible to you once you hover with your mouse over the little cog wheel in a post's tag list.
Note, that the hidden tag is not meant to be a privacy feature. Rather, the feature allows to hide tags e.g. organizational-note-tags from publication lists for beautification.
The hidden tags are not visible on tag clouds and post lists, but, you can e. g. query for a certain sys:hidden tag and still find all posts tagged with it. For really private information the private note feature of publications will be the appropirate choice.