Your inventory is organized in Greenbits by Product Types, Products, Packages/Lots/SKUs and inventory items. Every inventory item in your store will have associated Package/Lot/SKU, Product, and Product Type in Greenbits.
Product Types > Products > Inventory Items
For example, let's say your store sells concentrate cartridges (Product Type). You sell several cartridges of different strains from different vendors (Products). Over time, you'll receive orders of those Products (Packages/Lots/SKUs). You'll sell the individual cartridges from those Packages/Lots/SKUs in your store (Inventory Items).
Learn more about how these relate to each other: