A champion skin refers to an alternate appearance of a League of Legends character. They are often bought with RP from the official store and vary in price. Also, skins have different tiers based on appearance and rarity. They are not essential to play League of Legends but can deliver a fresh look for your character. Moreover, Hextech chests can be opened to receive skins that can be purchased with orange essence for free. However, the chests are randomized, so you can't select the skin you want.