If your budgie's wings are folded and slightly held away from it's body and shakes them it's probably a sign of stress or tension. If your bird keeps it's wings folded but away from it's body without the shaking it's most likely feeling a bit warm or hot. So he keeps his wings away from his body to cool down.
One of my budgies has done the wing shaking a couple of times. If your budgie doesn't show any signs of being sick or weak I wouldn't worry about it too much.
Hope this helped
Edit: It could also be a sign of pain but then you would see your budgie do it almost continuously.
Heat will cause them to lift their wings from their bodies, but not shake. I've heard of and seen quite a few that lift them and then the wing shakes/flutters, sometimes almost seeming uncontrolable. I personally feel its neuralogical, and not stress. I've had a couple do this over the years (in the aviary, with no stress to note), they are still living strong. Its a hard thing to have a vet diagnose, since its not done all the time, and of course never at the vets when you need it to! Since they have never shown any other signs of illness, it doesn't worry me as it did before.