Do you mean Tarr Unlisted? The only time you mostly see the term Unknown is when the Last Name At Birth is Unknown. Since you have the last name, Tarr, I can't think of where you came across this.
However, I think you mean the Tarr Genealogy Page that has all of the Profiles on WikiTree with the last name of Tarr? To get to this page, just click on your last name on your Profile Page. This will give you that list of Tarrs on WIkiTree.
Unlisted Tarr on that list means the person is still alive or their date of death has not been added and they were born less than 100 years ago. WikiTree cannot show the data of anyone it believes may still be living and thus they become Unlisted Tarr or SMith or Whatever. The Profile Manager of that unlisted person or anyone on the Trusted List for that Profile can see any data that has been added to that Profile, but we, the gerneral membership or public can't. It a legality ruling over Privacy Laws.
If that doesn't answer your question, please let me know by replying to this Answer.