Inventory refers to the system which manages the items that the player is carrying at any given time throughout the course of your game. This is represented visually by the inventory window as shown in the image below. When enabled, the player can access their inventory by pressing the “I” key.
The inventory system is enabled by default, but if your game doesn't have a need for it (or you don't want the visual aspects like the inventory button and window to be shown), you can disable it from your Game Configuration. If it is disabled, you can still make use of the player's inventory behind the scenes through scripting functions like Give Item or evaluating if the player has an item via a conditional expression.
The visual appearance of your game's inventory window and button can be customized from the Theme Editor by editing the images for “Window Titlebar”, “Window Body”, “Item Slot”, and “Inventory Button”. Most of these images are shared with the item container window.