It means that the studs that are on a ford have a different thread than those toyota uses. So all you need to do is get a new set of lugnuts that are for the rims but with toyota's thread size. Thus the ford rims will then bolt right up.
Stock Mustang wheels (and all the replicas that I'm aware of) use a conical-seat lug nut. If you have stock Toyota alloy wheels you need to replace the stock, shank-style lug nuts with metric (M12x1.5) conical-seat lug nuts.
If you have factory steel wheels your stock lug nuts will work.