An aircraft's Vne is the velocity that should never be exceeded. Generally, Vne is specified as an indicated airspeed in either knots or miles per hour. Most airspeed indicators have a red line indicating Vne for that particular aircraft.
Exceeding Vne risks structural failure. On some aircraft exceeding Vne results in a loss of control due to wing flexing. On some aircraft, stresses in the tail may be sufficient to cause structural failure.
Generally, an aircraft manufacturer first determines the speed at which the first obstacle to safe flight occurs even if the aircraft is unaccelerated and no abrupt control inputs are applied. This speed is then reduced to provide a safety margin. It is not safe to fly at or near Vne in turbulence or when abrupt control inputs are applied.