At high angles of attack the boundry layer will become 'unstuck' and the aircraft will stall. If you can imagine a large curlover of air, departing the top wing surface such that the curlover causes airflow at the trailing edge to be reverse in direction to flight, along the top surface. The vortex generators, being proud of the surface, cause a similar but smaller curlover in a lateral sence. If you wind a bit of paper around a stick then pull the ends apart you get a coil of paper like the coil of a spring, this is what the vortex might look like if you could see it. The airflow of the vortex has the effect of reaching up and grabbing the departed air and returning it to the wing and thus re-establishing a boundry layer, even though it is a turbulant one. This allows the wing to continue to fly when otherwise it would have stalled. When you fit high lift devices to the leading edge, the the wing will still fly but be uncontrollerble because the reverse flow at the trailing edge is in the wrong direction for the alerons to work. Vortex generators are thus normally place ahead of the alerons in this case so that the alerons will still function in the correct sense even into the stall, thus making the aircraft safer to fly at low speeds.