The main menu shows the entire page hierarchy of your website; the component you choose to present it determines where the menu appears and how it is styled. The main menu has two parts: the main part controls how top -level links are rendered and the flyout part controls how submenus are rendered. For most templates, the submenu is rendered as some sort of drop-down menu with rollover states; when you place your mouse over a menu item, a sub menu will appear (if there are child pages associated with the menu item). This sub menu disappears when you move your mouse off the menu item.
Note: You use the Reorder Pages feature to change the page hierarchy used for menu items.