The most effective SharePoint sites (and web sites in general) help visitors find what they need quickly so that they can use the information they find to make decisions, learn about what is going on, access the tools they need, or engage with colleagues to help solve a problem. Even when search is available, most visitors start their web experiences by navigating.

Now you can change the navigation style to increase the value of your site navigation. The new Megamenu options allow for multiple levels of hierarchy at once – no cascading fly-outs needed. This will enable you to better organize and showcase the content and sites. And of course, for sites that don’t need the megamenu look and feel, you can still use the Cascading option. You simply go into the Change the look edit pane, click on Navigation and choose between Megamenu and Cascading. You then add and adjust your navigation items inline as you had before – now with a new megatastic look if you choose.

https://techcommunity.microsoft.com/t5/Microsoft-SharePoint-Blog/Organize-your-SharePoint-sites-with-megamenu-navigation-and-new/ba-p/328068