Aerodynamics
As air flows over a typical wingspan the pattern of the flow depends very much on the angle between the overall direction of the upstream airflow and the shorter dimension of the wing. When this angle is a few degrees the air pressure on the upper surface of the wing is less than that on its lower surface and there is a net upward force on the wing (called lift) in addition to a retarding "drag" force opposite the direction of the upstream airflow.
As the angle of attack is increased the vortex shedding becomes more intense and the lift force deceases compared with the drag on the wing. If this happens in flight the lift may no longer sustain the weight of the aircraft and it "stalls". Unless lift is restored it will "fall out of the sky".
From an engineer's point of view an aircraft may be viewed as a deformable mechanical system composed of plates, fluids and gases. As it flies through the air it trembles and deforms just as a blob of jelly would if tossed into the air. The difference is that the normal mode spectrum of the aircraft is different from that of a blob of jelly and (thankfully) metal is stiffer than gelatine. Of the millions of natural modes of vibration of a modern aircraft the most important are the twisting and bending modes of the tail, wing, fuselage and appendages such as engines. The aircraft is designed with variable surface structures. These can be used to direct the pressure forces due to the movement of air over them in order to counteract the excitation of the "natural modes" of the aircraft. These surfaces structures are often controlled automatically by sensors that detect the attitude of the plane in space and the torques and forces on its members. In "fly by wire" control these sensors send information to a bank of control computers that activate hydraulic devices to actuate the necessary changes in the surface structures many thousands of times a second.
When aircraft were first designed such control mechanisms were pipe dreams and in order to keep aircraft stable in flight the distribution of weight of the aircraft and the geometry of the wing surfaces had to be carefully arranged so that stalling speeds were not too low and that instabilities induced by wind gusts etc were controllable by manned responses. This often necessitated sluggish aircraft performance in speed and manoeuvrability and poor fuel economy. With the development of high speed computing it became possible to admit naturally unstable aircraft designs into production since their instabilities could be controlled by rapid control of wing surfaces, rather like the way we discussed maintaining vertical stability of a rod on rapid oscillatory motion of one end. The bonus is that naturally unstable aircraft can be made lighter and much more manoeuvrable provided they fly within their design capabilities!.