Positions are the placeholders for modules in Joomla. The Gantry framework offers a plethora of module positions for you to choose from. This page has a default HTML module published in every available position except key positions like navigation.
The template uses a 12 column grid. All column positions are equally wide in this page because they are all being used but you can choose different widths if you use less columns. To have different layout configurations for different pages you can use the Joomla template style system.