This referes to the internals of Vaadin and the communication between the web browser and Java server. You don’t have to care about the nitty gritty details about the communication as it has been abstracted away to a nice Java API in the server side (and in the client-side to some extent). That’s the beauty of Vaadin.
To answer your question: The X of XML refers to the XMLHttpRequests made in Vaadin’s JavaScript code. See
wikipedia entry on AJAX .