It adds Ca, Mg, and SO4 ions to the brewing water. It will also lower the pH of the water. They are also known as Burton salts after the English town of Burton on Trent, renowned for the brewing of Pale Ales.
If your water is low in calcium or sulfates and you are brewing a pale ale, you could dissolve them in the mash water prior to mashing.
If you are using extract, and/or you don't know your water chemistry, then I would simply forget about them, as they may make the beer slightly better, or they may make it slightly worse. I also wouldn't use them if I were brewing anything other than a pale ale.