Inventory refers to your characters two storage compartments that consist of the Toolbelt and the characters Inventory. The Toolbelt can be seen while interacting with the environment. The Inventory on the other hand can only be accessed by pressing the TAB key. Items can be moved to and from the Inventory to the Toolbelt for easy access to items when exploring. When in your Inventory you can move, drop, repair, modify, or use items by hovering over them and selecting them and choosing the appropriate action keys. In the Inventory the player can also access and interact with the Crafting Menu.
When starting a new game the amount of available slots in the characters Inventory will be limited for full use until either the player uses clothing or armor Mods to expand it or by using Perks to unlock those slots to eliminate the side effects. Without those, filling these slots with items will limit your characters Stamina and increase the amount of time the character becomes fatigued. With every item addded into these slots will increase the lack of Stamina.