VG's are small parts about few square inch/each fixed on the extrados of a wing.The meaning of VG's is to create vortices which will transfer energy from the general flow of the air towards the layers that sustain the most "braking effect" close to the surface of the wing.In other words the time the flow will stall will be delayed thus the friction drag is lowered so will be the fuel flow.The general idea is to transform laminar flow into turbulent (more energy).The ratio between inertia/viscosity of a flow is called Reynolds number(Re).In this relation you can see that if you reduce the viscosity, Re will increase:the higher the number the more turbulent the air flow will be thus the more energized and the less friction agaisnt the wing.This is exactly the purpose of VG's.
Hope I could help
