The standard approach to menu dropdowns has each sub-category appearing under main menu when the top-level menu item is hovered over. With the Megamenu, all items from two levels deep are shown on hover with the menu stretching full width across the menu width.
There are two Mega menu formats. The first shows categories two levels deep, while the second mega menu shows only one level deep, with the second level showing on hover. This is useful for sites that have many nested categories