This creates a menu ($menu::menu)showing the available subdirectories from your current position. A subdirectory, or one of its “descendents” must have a story in it in order to be shown in the menu (ie, it does not lead down “blind alleys”). Additionally, it abides by the excluded sections if the exclude plug-in is enabled.
The default configuration settings at the begin lead to a sort of reversal of the breadcrumbs plug-in.
Additionally, the $menu::rootmenu variable contains the top level directories, regardless of the current page.