The VisualEditor is a way to edit pages, well, visually, as opposed to editing the source code. The VisualEditor is activated when you press the "Edit" button on a page, while the source editor is activated when you press "Edit source".
VisualEditor support is only enabled on pages in the Main, User, File, Category, and Project namespaces. In other namespaces, you must use the source editor.
For more information on how to use the VisualEditor, click here.