It is Peco lingo and means that the frog area of the turnout is electrically dead (insulated) and does not make contact with anything else. This ensures that the (metal) wheels of any passing rolling stock cannot create an electrical short. On the other hand it can create a pick-up problem if the insulated area is to long.
Compare this to an electrically connected frog (electrofrog in Peco lingo) where the pickup problem is avoided but where the electrical polarity of the frog must be changed as the turnout is switched.
