The cardboard box is the least expensive crate you can purchase in Book Of Monsters. Boxes like these will award skins or costumes that change the appearance of your monsters. You can choose which monster to buy it for.
The cardboard box costs 500 Coins or 40 Dux, and will give you one skin. It has a 94% chance of giving you a common skin, a 5% chance of giving you a rare skin, and a 1% chance of giving you a legendary skin. You can also get this on day 1 of your daily rewards.