Load On Demand means that during the initial load, only the content of the selected tab being displayed in the ContentPane is requested from the server. If the user doesn't select any other tabs, this gives them a much more responsive user interface because they never needed the content residing in your other tabs.

When the user selects another tab, then two things happen. First, WebTab™ caches the content on the tab the user just looked at (figuring that there is a good chance the user may return to it later, and thereby saving a request in the future). Second, the content for the new selected tab is requested from the server so that it can be displayed in the ContentPane.