Plain English Meaning
The fastest you should fly when the air is bumpy. Going faster than this in turbulence risks damaging the airframe because a strong gust hitting the wings at high speed creates loads the structure isn't designed to handle.
Full Definition
The highest indicated airspeed at which an aircraft may be operated when flying through turbulent air. It is set below the structural cruising speed to ensure that a sudden gust load will not exceed the aircraft's structural limits. This speed is commonly published in the Pilot's Operating Handbook as Vₐ (maneuvering speed) or Vₙ₀ (maximum structural cruising speed in rough air), depending on the aircraft.