Mod_submenu is a Joomla module that displays a list of submenus and "leafs" of a given menu. The list entries have hyperlinks for the visitor to click.
The module is especially useful if you define empty (so called "text separator") menus, which have no other content assigned, but have submenus. These menus usually can not be clicked - which somehow is irritating the visitor.
While mod_submenu automatically generates the menu link list at runtime, also the corresponding "breadcrumb" (and other) menu items will display useful contents when clicked.
In the picture above you see the pulldown menu "More…" opened. The corresponding menu list (blue) is shown on the right side. Of course, you can assign a mod_submenu instance to a menu item together with other content or alone.