A mega menu is a multi-level menu that comes with drop-down interface and contains navigational links to archives and categories.
If you're looking for a Magento 2 mega menu extension that is easy as 1-2-3, then Ninja Menus by Magezon is a great option.
https://www.magezon.com/magento-2-mega-menu.html
Using Ninja Menus, you're not limited to one type of menu. Instead, there are 4 types for you to choose.

1. Horizontal Menus
With this kind of menus, the menu items are displayed horizontally, like this:


2. Vertical Menus
Display menu items vertically:


3. Accordion Menus
Accordion menus allow you to expand submenus like an accordion:
[img]https://www.magezon.com/pub/media/ninjamenus2/menu-types/Ninja-menus-accordion-menu.gif
[/img]

4. Drill-down Menus
With this menu type, when you click on a parent menu item, it will slide to display the children menu item.