NBT
Certain things in Minecraft internally use the NBT datastructure. An NBT tag is a special kind of object that can hold multiple values of different types. For example the NBT representation of a Chest containing all its items, the enchantment information of an Pickaxe, ...
Variables can hold references to such NBT tags, and corresponding operators exist to read or extend these tags.
NBT tags are typically used in this mod as a low-level fallback mechanism, in case you can not read certain information using dedicated operators.