That means you don't have to pull out your phone to check your messages while you're walking around a city during your morning commute. You can also respond to the text messages using your voice, and Siri will send them off for you. This is a feature that will roll out in iOS 13 this fall, but I've been using the developer beta so I can show you how it works.
And you'll be able to try it for yourself soon. Apple plans to release its public preview of iOS 13 sometime in July, so bookmark this page to follow the steps when it's available.
That's it. Now, when your phone receives a text message, Siri will speak the contents of that message into your AirPods. It seems like Siri should be able to do the same for notifications from other apps in the future, but right now it's purely limited to messages.