As a normal documentation author you do not need to know anything about Help+Manual's XML source code to get all the benefits from using Help+Manual. Familiarity with XML is really only needed for users who want to generate their topic files from their own applications automatically in XML format and for performing complex search and replace operations directly on your project files with an external editor.
If you are used to editing HTML pages you might feel tempted to try to edit the XML code in the XML Source tab to tweak your topic formatting. Because of the absolute strictness of XML, doing this will be more trouble than it is worth in most cases. It is best to use this tab just as a reference when you need to know how your topic formatting is defined in the XML source.
XML editing is for experts only. Although XML looks a little like HTML it is actually very different and does not tolerate even the smallest syntax error.