Reward Tables are required for Random, Loot and Choice rewards, as well as Loot Crates. You can create a table in the FTB Quests settings page, accessible from the main GUI.

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 first create a Reward Table and then change its  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 Commons weight for animals to 2000 and Rares to 1000. They will now drop in a ratio of 2000:1000, so Common crates will drop twice as often as Rare.