The path defined by the ``blog_show`` route acts like ``/blog/*`` where

the wildcard is given the name ``slug``. For the URL ``/blog/my-blog-post``,

the ``slug`` variable gets a value of ``my-blog-post``, which is available

for you to use in your controller (keep reading).

for you to use in your controller (keep reading). The ``blog_show`` is the

internal name of the route, which doesn't have any meaning yet and just needs

to be unique. Later, you'll use it to generate URLs.

The ``_controller`` parameter is a special key that tells Symfony which controller

should be executed when a URL matches this route. The ``_controller`` string