Past participle of “bieten”, “to bid”, as in “to command”. English correspondence to that is “bidden”. The prefix is “ver-” which here denotes completative aspect as well as negativity, the English correspondence to that is “for-” (not strict, but at least in this case). Sticking both together you get “forbidden” which indeed is the right translation. “The action of commanding a negative has been completed”, or, simply, “You’ve been told not to”.