This effect adds glass bevels to the edges of your photographs, to make them look as if they are under a modern glass frame:
Most of the settings are self-explanatory except for distance. What that does is, when it is at a low setting, is give a more variable shading over the length of the bevel. Compare these two screenshots (look at the distance setting and the left edge of the image):
The difference is only really visible in image which do not have much detail on the edges.