It's CarbonX non-flammable fabric. National Geographic's The Showdown of The Unbeatables explains:
CarbonX is an acrylic fiber that is partially charred in an incinerator, a carbon sheath forms around an unburned acrylic core. When this charred fiber is exposed to heat, the acrylic core burns completely, absorbing the heat energy of the flame.
The fabric can dissipate energy and deflect the heat away, all while being incredibly thin and light. It's used by race car drivers and fire fighters but I think it's basically magic material from the fire gods.