When we talk about vignetting in photography, we are normally referring to the darkening of the corners of the image frame due to light blockage from the edges of the lens housing, and the chances of this usually increases with the number of filters etc that you add onto your lens. In extreme cases, you can get a dark circle around the image.
(As an aside, the proper definition of vignetting doesn't always refer to the above undesirable effect. A vignette is a kind of design whereby the edges of the image fade off into its surrounding background, so "vignetting", properly speaking, simply refers to creating such an effect.)
