When working on mobile devices, one of the things that you'll need to spend more time on is the user interface, or UI for short. Unlike when developing projects for PC, where you need to only care about a single resolution or aspect ratio, you should keep in mind that there are many different devices out there when building for mobile. For instance, we have phones, which fit in our pocket, but also tablets, which are huge. Not only that, but games can also be played horizontally or vertically.
A Graphical User Interface (GUI) is the way that players interact with your games. You've actually been using a GUI in all of the previous chapters (the Unity Editor) and also when interacting with your operating system. Without a GUI of some sort, the only way you'd be able to interact with a computer is a command prompt, such as DOS or UNIX.
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