As per the refurb guide:
New Open Box – This product has never been used. The product has been opened to add, remove, or replace items such as warranty cards, user manuals, or accessories. The item may come in a retail box but could ship in a plain box. New Open Box items come backed by a manufacturer or vendor warranty OR our very own Woot! Warranty.
(Please note: I don’t work for Woot, I just volunteer to help out here on the forums.)