Vignetting is when a filter, lens or editing software makes a dark boarder in the corners of the image. This is bad when you are using filters or an add on lens. It can be cropped out to improve the image.
In the example I have shown the camera which has a 49mm thread on the lens. To the camera we have add a UV filter and a Polarizer lens. You can see that there is vignetting on the output image.
In this case if we add a 49mm to 55mm setup ring and then use 55mm filter we can eliminate the problem.