There are a couple ways that I use those the most basic is to set that layer to 'screen' or 'lighten' over an image you are using to make an icon. It pretty much "nullifies" the black part there and reveals the image underneath. Like this:


There are other things too - literally turning that into a mask layer over another texture to create other effects, and I believe you can find tutorials for that (and other things) online if you google