I'm guessing — but that's really a guess with no knowledge of the thing — it's some sort of middleware thingie: the "plugin broker" is used to register plugins, and in this case any controller inheriting from `Zend\Mvc\Controller\ActionController` will get the same broker, therefore all plugins registered into `Zend\Mvc\Controller\PluginBroker`. I'm guessing it also handles calling the right plugins at the right moment in the controller's lifecycle. Something like that.
Awesome phrase.