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.
By default they have empty loot tables, so opening one won't do anything, but you can change them in
Quest GUI -> File Settings -> Loot Tables or you can tweak existing, blank tables with Loot Tweaker mod.
Info outdated, will be updated soon. Basically, you create them from Reward Tables