This hook enables Drupal modules to register paths in order to define how URL requests are handled.
As defined in Drupal API Docs:
This hook enables modules to register paths in order to define how URL requests are handled. Paths may be registered for URL handling only, or they can register a link to be placed in a menu (usually the Navigation menu). A path and its associated information is commonly called a "menu router item". This hook is rarely called (for example, when modules are enabled), and its results are cached in the database.