The router creates and interprets nice Search Engine Friendly URLs
The router translates nice Search Friendly URLs like
/some-name/some-other-name to internal links like
/index.php?option=com_component&view=some-view&id=123 that Joomla uses to open a component + view + specific item.
The router also creates (builds) nice URL when displaying such internal links on the front-end of the website