These are chests (or packs of cards) with random content that are offered in Free-to-play games (Hearthstone, Roblox …), but also now in games paid full price (Overwatch, Star Wars Battlefront 2, FIFA 21). So you can’t be sure of getting the right object and you may never get the right object after spending a lot of money, because of the randomness.
This practice is much maligned in the industry and by gamers but unfortunately still very present in today’s games. Some governments such as that of Belgium have banned loot boxes in games that come out in their country.
