From Joomla! Documentation
A split menu is where different levels of a single menu are displayed in two or more locations on a single web page.
For example, a common requirement is for a menu of top-level items to appear at the top of the page. When one of the items is clicked the user is taken to a page where a secondary menu, say on the left of the page, shows second-level items within the scope of the top-level item.
The menus appear in separate locations on the page, but are related because one shows only top-level items while the other shows second-level items. This idea can be extended to include menus for third-level items and beyond.
This can be implemented in Joomla using a single multi-level menu then creating more than one menu module each referring to a different level.
See also: Menu