A skin is a visual display style for a Prismata unit. Each player by default has the Classic skin equipped for all units, but alternative skins can be won in the Armory. Once you have won a skin, you can choose whether to equip it in the Skins tab of the Customize section. If you do equip the skin, all of your units (but not your opponents units) will be displayed with that skin in games. Many units have multiple alternative skins. If you win more than one skin for a unit, you can only choose one to equip (or you can keep the Classic skin).