How do priorities work in the *.services.yml YAML file?
Drupal uses the EventDispatcher from Symfony and there the priority is used to control in which order the event listeners are triggered:


  An optional priority integer (higher equals more important and
  therefore that the listener will be triggered earlier) that determines
  when a listener is triggered versus other listeners (defaults to 0).
  If two listeners have the same priority, they are executed in the
  order that they were added to the dispatcher.


So the priority in D8 is exactly the opposite of the weight in D7.

The example you used in the question is not an event subscriber, but it uses the same concept of priorities.