XML (eXtensible Markup Language) is used in many aspects of Java development—including Maven Project Object Model (POM) files, XML-based properties files, configuration files for Java servers and deployment artifacts, layouts for JavaFX and Android GUIs, etc.
For the most part, the basic rules of XML syntax—i.e. what constitutes well-formed XML—are very simple, but strict. They may seem excessively strict if you have previous experience with HTML, but not XML; the former is much more forgiving than the latter. Beyond that, there are features (particularly DTDs and schemas, which specify what constitutes valid XML under specific conditions), that add complexity to the understanding and use of XML. Nonetheless, the great majority of XML usage can be understood without too much difficulty.
Please read the page at the first link below in its entirety, and as much of the second as you have time for (it has lots of rabbit holes that I don’t expect everyone to dive into). These are also linked from the resources page.