The main navigation located in the header displays the first level links assigned to your navigation tree.  "Mega Menus" appear when hovering over a main navigation item.

A mega menu is divided into two parts.  The left column is the section and the right column is the sub navigation for the corresponding top level link (2nd level links).

The custom content is added to mega menus through  the "Sections" manager. The "Sections" manager can be found under the "Content" tab in the main navigation.

A naming convention is used to associate content to nav items.

The bottom column of a mega menu looks for a "Section" titled "Nav Section [nav position integer]", where "[nav position integer]" is the numerical position of the top-level link, beginning from the left.

For example, the Next Steps link from the top screen shot would have a section labeled "Nav Section 4".

The section background image is used in the left column with the section wysiwyg in the right column.