This means that one cannot precisely predict the value of a flow property at a future instant by any known means, since the precise relationship is not known. In the boundary layer over a large wing in flight, the velocity at a given point cannot be predicted for every instant, because it keeps fluctuating. We can watch the fluctuations for as long as we care we will not be able to predict the exact value for a future instant. Every time the blade comes around, the flow velocity undergoes sharp fluctuations. Turbulent fluctuations occur over many frequencies.
Fluctuations occur in properties along all directions.
Cascade of scales. Even turbulent flows must obey the laws of physics. Therefore, the momentum and continuity equations must hold, on an instantaneous basis, in relating the properties of the flow. The trouble is, all the properties keep fluctuating from instant to instant. We develop approximate means.