We already know that we will write Kotlin code that will use other people's code, and that will be compiled into DEX code that is used on our users' Android devices. In addition to this, we will also be adding and editing other files that get included in the final APK. These files are known as Android resources.
As mentioned earlier in this chapter, our app will include resources, such as images, sound, and user interface layouts, that are kept in separate files from the Kotlin code. We will slowly introduce ourselves to them over the course of the book.
They will also include files that have the textual content of our app. It is convention to refer to the text in our app through separate files because it makes them easy to change, and easy to create apps that work for multiple different languages and geographical regions.
Furthermore, the actual User Interface (UI) layout of our apps, despite the option to implement them with a visual designer, are read from text-based files by Android.
Android (or any computer) cannot read and recognize text in the same way that a human can. Therefore, we must present our resources in a highly organized and predefined manner. To do so, we will use Extensible Markup Language (XML). XML is a huge topic; fortunately, its whole purpose is to be both human- and machine- readable. We do not need to learn this language; we just need to note (and then conform to) a few rules. Furthermore, most of the time, when we interact with XML, we will do so through a neat visual editor provided by Android Studio. We can tell when we are dealing with an XML resource because the filename will end with the.xml extension.
You do not need to memorize this, as we will constantly be returning to this concept in the book.