Darkens the corners and edges of the frame, the way a camera lens does when it cannot quite cover the film. Photographers spent decades trying to avoid it and now add it back on purpose, because it quietly pulls the eye toward the middle.
Every pixel measures how far it is from the centre, scaled so the corners come out at exactly one, and that distance is squared. The squaring is what gives a vignette its particular look: most of the frame stays barely touched and the darkening rushes in near the edges, rather than fading evenly the whole way out.
brightness is the colour the edges head toward, and it does not have to be black. Set it brighter than the picture and the effect reverses, closing a pale surround in around a darker centre. alpha sets how strong the whole thing is.