This module displays one of the site's menus on the front end. A single menu can be shown in more than one place, and a site can run as many menus as it needs — a main navigation, a footer list, a sidebar for one section only.
The module chooses which menu to show, which item to treat as the starting point, and how many levels deep to go, so a sub-navigation for one branch of the site is just the same module pointed at a different base item. How the result looks — horizontal bar, dropdown, stacked list — is the template's job, handled through its CSS and layout overrides rather than through the module.