Skins are cosmetic enhancements for your weapons, that you can select from the loadout menu, to give your weapons a unique look.
You can purchase skin cases in the shop, grind Critical Pass, buy bundles, or spin the Lucky Spin to get skins. The type and rarity of the skin you get depends on the type of case that you open. Skins are also available to be purchased directly from the store with credits when they are on offer as a flash sale.