With real money, you can purchase credits, the currency that is used to purchase in-game items. Credits can also be earned by milestones, the Critical Pass, watching ads and completing offers.
With credits you can buy cases that contain skins for the weapons. The skins only offer a cosmetic enhancement, they do not change game play in any way. Cases contain a randomly determined skin, if you obtain a previously owned skin when opening a case, you will receive blue tokens, a currency that can be used to buy skins directly from the loadout menu.