The inventory category is about blocks that have inventories. Inventories are, at their root, containers for storing items. Medieval Engineers uses inventories in various ways including to visually display stored items and as a place where items can be crafted into other items. The player's inventory and crafting screen can be accessed by pressing and all external inventories, including crafting stations, can be accessed by aiming at them and pressing .
See Inventory and Crafting for more information about how to use inventories.