Loot Crates are items that when broken by right-clicking, will give you loot. Usually there are 5 different crates, but there can be custom ones. Commonly available rarities: Common, Uncommon, Rare, Epic, Legendary.
To create a loot crate, you create a Reward Table and change it's Loot Crate setting to true. You can adjust drop weight for each entity type. For example, if you have 2 loot crates, Common and Rare, you can set Common's weight for animals to 2000 and Rare's to 1000. They will now drop 2000:1000, a.k.a 2:1, a.k.a Common will drop 2x more than Rare one.