When descending with a headwind, the aircraft will be able to achieve a steeper descent path over ground (flight path angle) than in zero wind or tailwind conditions, because ground speed reduces with increasing headwind, while rate of descent remains constant.
If the FMS is programmed with say no wind, it will calculate a somewhat shallow descent path. If you then try to follow the calculated descent path in a headwind situation, the airspeed will reduce, unless thrust is added.
Hope that makes sense.