A vignette is an artifact caused by light falloff at the edge of the image.
Photographers sometimes add a vignette the post-production phase to direct the viewers eye within the image. The added effect can be darker or lighter than the central image. A lighter vignette gives a dated effect to the image and can be distracting on images that are not already high key.
Vignetting caused by the lens is often undesirable and needs to be corrected in post-processing.