Bring focus to your subject.
Vignette is a reduction of an image's brightness or saturation at the periphery compared to the image center. The word vignette, from the same root as vine, originally referred to a decorative border in a book. Later, the word came to be used for a photographic portrait which is clear in the center and fades off at the edges. A similar effect occurs when filming projected images or movies off a projection screen.