It all starts with a route, doesn't it? Most interactions with a Drupal 8 website begin with a user (or system) accessing a certain path (or resource). This translates into a route, which maps that resource to a flow that (hopefully) returns a successful response back or at least a graceful failure.
The Drupal 8 routing system is a major shift away from how it used to be in its previous versions. In Drupal 7 and before, the routing system was a very Drupal-specific thing (a drupalism, if you will). Many of us remember hook_menu as a staple hook each Drupal developer had to know very well. All of that has been abandoned in Drupal 8 in favor of the Symfony Routing component (http://symfony.com/doc/current/components/routing.html). Also, since I mentioned hook_menu, I will also mention that its other main functions have also been taken over in Drupal 8 by other subsystems, such as plugins.
In Chapter 2, Creating Your First Module, we will see how we can define our own route and map it to a controller that will render our page. We will cover a few of the more important route options and take a look at how we can control access to these routes.