What that means, simply , is to cover the small gap at the front edge of the control surface to prevent airflow between the control surface and the wing (or tailplane). This helps reduce flutter at high airspeeds. You can come from underneath the surface ( more aesthetic) or go on top with a very carefully cut piece of whatever your plane is covered with and iron it on. Either way, be VERY sure that this strip does not impede control surface travel in either direction.