Complex sentence: The primary aerodynamic requirement on the cross-sectional shape of the wing is the need to keep air flowing smoothly over the entire surface of the wing to generate the required lift and to minimize turbulence (which creates parasitic drag and other problems).
Split into simpler sentences: The primary aerodynamic requirement on the cross-sectional shape of the wing is to keep air flowing smoothly over the entire surface of the wing. This is to generate the required lift and to minimize turbulence (which creates parasitic drag and other problems).