In the previous chapter, we discovered how to interact with game objects in the world space scene. Not only can these objects can be balls and toys, or tools and weapons, but they can be buttons you interact with and other user interface widgets. Furthermore, Unity includes a user interface canvas system for building menus and other UI.
Graphical user interface (GUI) or just UI, usually refers to on-screen two-dimensional graphics, which overlay the main gameplay and present information to the user with status messages, gauges, and input controls such as menus, buttons, sliders, and so on.
In Unity, UI elements always reside on a canvas. The Unity manual describes the canvas component as follows:
The canvas component represents the abstract space in which the UI is laid out and rendered. All UI elements must be children of a GameObject that has a canvas component...
