A lootbox is a container with randomized loot.
Unlike chests or in-game items, this container is usually bought or given to the player directly. A player buying a lootbox is essentially gambling resources towards acquiring random items.

Having lootboxes changes the game substantially: presenting an alternative in form of pay2win concept replacing actual gameplay(gaining loot, finding random items, experience vs buying loot, gambling and experience-giving items ) makes cash shop central to the game: balance is shifted towards players spending money vs free players.
More: https://en.wikipedia.org/wiki/Loot_box