aka 'Child Menus', 'Page Hierarchies', 'Nested Menus' and a lot of other things
In Page Options you can choose a 'Parent' to mark a page as the 'Child' of another. This has the effect of giving the page a url of example.com/parent/child but doesn't alter the navigation menu links. GS supports any number of levels of url hierarchy, but it's native menu function only displays a single, simple list of links.
get_navigation() template tag creates a simple, single series of links to pages marked for inclusion in the menu arranged in order as
<li> elements. To generate nested lists or to separate different levels for different menus or to display page titles instead of the specified menu text a plugin is used. I18N is the most popular plugin or there are more simple plugins also in extend. Advanced users can create their own custom menu functions.
Not all GetSimple themes have CSS to display more than a single
<ul> as a nav menu. You can style your own menus or can to choose a theme which supports multi level navigation.