An item container is an object that has been defined as such from its Entity Properties dialog in the Map Editor. This functionality allows an object (for example, a treasure chest, bookcase, or mailbox) to act as a container from which items can be retrieved. It can also be used as a way of allowing the player to store items in an object, for example a personal storage box in their house.
The visual appearance of your game's item container windows can be customized from the Theme Editor by editing the images for “Window Titlebar”, “Window Body”, and “Item Slot”. These images are shared with the inventory window.