Borders, Overlays and Masks
This graphic shows an example of what a border, overlay or mask will do. You don't have to use those mask layers as specifically intended. I often use them in just a layer mode setting for effects more than actual editing and selecting.
The black and white image is the border used on both photos. The one on the left is used when the image is black with a white border, the one on the right is white with a black border. Note the gray edges and how the yellow coat pops and his face is highlighted.
Many are frightened about using overlays like these because they don't know how to use them and in their raw state they look kind of ugly. Don't fret. If there are items on the overlay you dislike, do some brush work, even eraser work, smudge it, blur it. Use invert, rescale, pixelate, noise.