Benjamin Mayo, 9to5Mac:

You need to be an iPhone and AirPods user to take advantage of the feature. With the AirPods in your ears, you will be able to listen and respond to incoming text messages. Siri transcribes the message so you can hear the text without looking at your phone or watch. You can then ignore or immediately reply without having to say ‘Hey Siri’.

The sense I get is that the H1 chip is required here, so the second gen AirPods or Beats headphones with H1 chip required.

The incoming message alert is accompanied by a special alert tone that differs from the regular text tone. Siri will introduce the message with a phrase like ‘Zac says’ before reading the message. If another text comes in from the same sender, it will adjust its description to naturally continue the conversation.

Time will tell if this feature makes it all the way to the public iOS 13.2 release.