In photography and optics, vignetting is a reduction of an image's brightness or saturation at the periphery compared to the image center. You can use it to draw attention to the center of the frame. (from Wikipedia)
Here is an image without the effect:
and the same image with the effect applied:
The built-in vignette effect has the following attributes:
- Offset: Controls the offset of the effect.
- Darkness: Controls the darkness of the effect.