Now you have a mastering_drupal_8.services.yml file with:

services:
 mastering_drupal_8.weather_underground:
 class: Drupal\mastering_drupal_8\WeatherUnderground
 arguments: ["@http_client"]

The first two lines are pretty simple: we give our service a name, namespace it to our module, and set the class that is used to instantiate the service. That third line defines the arguments that are injected into the service. In this case, since we're going to be making calls to an external API, we need to have an HTTP client. Those two attributes, class, and arguments, are the most common metadata in the services.yml file.

Other, less common, options include: