Adding a vignet involves the lightening or darkening of the edges of a picture to draw the viewer's eye towards the center. Most lenses have a natural vignetting effect as the lens darkens towards the edges and lens correction software can be used to remove it in post-processing.
Adding a vignet seems particularly effective for portraits and works best when the subject is fairly central and the effect is applied in a subtle way. Many types of picture can benefit from having a vignet applied, but it shouldn't be done routinely. I'd suggest it is only used when there is a strong subject rather than in the case of a general landscape shot. Experiment with the effect and see what works best for you.