Air flows over and under the wings of an aeroplane as it moves. Wings are designed to make the air travelling over the top surface move faster than the air moving under the wing, and this has the effect of lifting the wing up.
The shape of a wing viewed from the side is known as an aerofoil (or airfoil), and the angle between the aerofoil and the direction in which the plane is moving is described as the angle of attack. The air flow separates at the leading edge of the aerofoil.
You can see the effect in this diagram of the air flowing over the top surface speeding up, and air flowing below the aerofoil slowing down. The coloured bands represent air molecules that were initially in the same relative vertical position. The video below demonstrates the same effect when bands of white smoke are injected into the air flowing above and below an aerofoil.
Increasing the angle of attack makes the air flow even faster over the top surface, and creates more lift. But if the angle is increased too much then the air stops flowing smoothly over the aerofoil and becomes turbulent.
The sudden loss of lift in a stall makes an aeroplane difficult to control, and usually led to a crash for the early pioneers as they were flying close to the ground and would not have time to recover. Otto Lilienthal died in this way, and in order to avoid a similar accident the Wright Brothers adopted a design for their planes that made recovery from a stall as easy as possible.
Wright Brothers test glider of 1900 after an accident.

It wasn't until 1901 that Étienne-Jules Marey developed a photgraphic technique, using thin streams of smoke, that allowed the air flow around different shapes to be visualised for the first time. And 1903 before Ludwig Prandtl began the series of experiments that provided for the first time a proper understanding of air flow and a scientific explanation of how the aerofoil can stall when the airflow separates from the top surface.

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