In the thermal underwear, some people say that they sewed a golden nail sticker. What is this golden nail sticker? What's the role?

Golden nails mean that a part of clothes is covered with yellow fluff, usually on the back, abdomen and knees of arms and joints. These parts that are afraid of cold are covered with golden nails, which is equivalent to thickening, which is much thicker than the uncovered parts. Just as our feet are afraid of cold, we put insoles on them.