The in-game skins icon

Skins are one of the secondary resources of your Civ. They are gained as a secondary resource while gathering food, both through farmers after the obtaining the Skinning upgrade and through manual clicks at any time. The Trader may also request skins in exchange for gold.

Skins can be further made into leather by a tanner, which requires that a tannery be built.

FoodWoodStone  –  SkinsHerbsOre  –  LeatherPietyMetal  –  Gold