The theme utilize TB Mega Menu module for creating mega menus. To create or edit a Mega Menu go to Structure > TB mega Menu under Administration toolbar. (
To make a certain block visible under the TB Mega Menu "Blocks" dropdown list, you have to place it in one of the theme's regions. You can also disable the block so it will not appear on your page.
Let's say you have a view that shows three most popular products in the Store, and you want to place it in the Mega Menu, under the Shop menu item.
The video below, will help you to better understand TB Mega Menu, its functionalities and how to use it.
Note that the video is referring to Drupal 7 module which is different then Drupal 8. However, the toolbox area, the TB Mega Menu simulator and the way that the menu is build are almost identical in both modules.