When you skin an animal, you have two choices. Cure it, or tan it. If you just let the skin be, it will rot. You can cure it by coating it in oil, fat, or salt. That will turn into a cured hide which can then be used like other fabrics. Alternatively you can soak the skin in a solution of quicklime (ground limestone dissolved in water.) Then you soak it in a barrel of weak tannin, followed by strong tannin. That becomes leather. There’s a few more details, but that’s the gist.