Plain English Meaning
It is a shudder or shake the pilot can feel through the airframe and controls just before the wing stops producing enough lift. It is the airplane's natural way of warning the pilot that a stall is about to happen.
Full Definition
A noticeable shaking or vibration of the airframe, control surfaces, or controls that occurs as the wing approaches its critical angle of attack, caused by turbulent, separated airflow coming off the upper wing surface and striking the tail or other downstream structures. It is one of the earliest aerodynamic warnings that a stall is imminent.