What does exactly mean "routing"?

What does exactly mean in the Drupal programming/developing language “routing definition” ? I have read following suggestion for trying to create a “Dynamic Title” as a controller plugin with Drupal 8: How do I set a page Title? -> http://drupal.stackexchange.com/questions/181828/how-do-i-set-the-page-t… I guess that to have[…]