Vortex Generators are little tabs the stick up from the wing surface. Their function is to mix the lower velocity air of the boundary layer with the higher velocity air outside the boundary layer. This helps to recharge the boundary layer and delays separation of the boundary layer from the wing top.
As the angle of attack increases the boundary layer (of air) begins to separate from the wing top starting at the trail edge. As the the angle of attack continues to increase the boundary layer continues to separate moving toward the leading edge of the wing.