As explained [[Wikipedia:VisualEditor/Portal/TemplateData|here]], TemplateData is a way of storing information about a template - such as parameter names, or a description of the template - so that the VisualEditor can retrieve it and populate the template editor with it. TemplateData is controlled by MediaWiki's [[mw:Extension:TemplateData|TemplateData extension]], which allows users to write small bits of structured data to a template page, or to be transcluded into that template page (such as on the standard documentation page). Once a template has this structured data, it can be displayed properly in the VisualEditor. While this may sound complex, it's actually very easy.
