The Drupal 8 routing system builds a great deal on the Symfony framework. To define and use routes, you don't necessarily need to know the Symfony concepts, but for more advanced use cases, knowing the background helps a lot.
Drupal 8 uses the Symfony HTTP Kernel. That is a system which gets the request and asks other systems to produce the requested output (a response object) and sends the response back to the client. The output is generated by piece of code called the controller. In theory the controller can be either a pure PHP 4 like function, a method on an object or even an anonymous function.
Symfony routing compared to Drupal routing
As module developer you define the list of routes and the corresponding controllers, for example:
Most Symfony documentation mentions pattern, but Drupal decided to just allow the non-deprecated 'path' key in its routing file.