Coefficient Drag is coefficient resistance for somethink, this is depended by velocity, area, viskositas fluid.
Cd = D/2.A.p.v^2, so we can estimated how many resistance for somethink in fluid, fluid can mean water and air. For example is someone use motorcycle, he is sit down and the else is stand up, so coefficient drag more big to stand up compare with sit down.
This application for calculate the plane, beside we must know Cd, in plane have lift that is given a Cl (coefficient lift). Back to Cd, so more speed is more resistance. This phenomena is used to handle the flow of plane. But from vector Lift and Drag make a angle. This angle is depend to up and down the plane.The wing of plane have foil, and this foil can change area and aa
The example is ball of golf, this ball is made not smooth, because to reduce Cd so this ball can fly without too big velocity is reduce. More details is every thing which fly in air have Cd and depend of wake from it. so if we can reduce wake, we can reduce Cd, the result we can get a big velocity.