Basically the range of airspeed in which both subsonic & supersonic airflow exist around a body ,in this case a propeller blade. The various parts of the prop(from tip to root) will experience supersonic airflow at different speeds, dependant upon the varying curvature & thickness-chord ratio .Broadly speaking it can be taken as MACH 0.8 to MACH 1.4 .This region of flight can be problematical due to somtimes extreme aerodynamic effects ,which can change control & Flight characteristics. In a prop this can cause vibration ,flutter ,fatigue,very high noise levels & in extreme cases a critical failure in the prop or associated components.