This is a conversion operator. It means that you can write this code:
XmlBase myBase = new XmlBase();
XElement myElement = myBase;
And the compiler won’t complain! At runtime, the conversion operator will be executed – passing myBase in as the argument, and returning a valid XElement as the result.
It’s a way for you as a developer to tell the compiler:
“even though these look like two totally unrelated types, there is actually a way to convert from one to the other; just let me handle the logic for how to do it.”