Steelskin is a gear perk that greatly raises your fire defense but slows you down a little. It is the perk to wear when you face flamethrowers or play with fire yourself.
Fire hurts you much less, so burning ground and incendiary ammo stop being a big threat. You move a bit more slowly in return.
Wear the Bombero Suspenders or Fuego Boots MK II. Gear perks are fixed to the piece of clothing that carries them, so you get this one by wearing that piece, not by unlocking it in a menu.
Fire builds that use incendiary ammo, molotovs or the Fuego Loco Supremo. You can fight inside your own fire with less risk.