Hello Martin,

The main_object variable holds the view object of currently rendered template.

Variables main_object.website_meta_description and main_object.name (additional title) can be set via "Promote" popup which can be access via the admin toolbar (black bar on top when logged-in via admin user) by clicking on "Promote" link.

The fields will be located under the "2. Reference Your Page" section in "Promote" popup.