We already know that we will write Java code that will itself use other people's Java code and will be compiled into DEX code that runs on the DVM 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.
Our app will include resources such as images, sound, and user interface layouts that are kept in separate files from the Java 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 makes it 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 actually read from text-based files by Android.
Android (or any computer) of course 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, but 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 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 extension .xml.
You do not need to memorize this as we will constantly be returning to this concept throughout the book.