Hot topic in SEO right now, Google put this out in beta in 2018 as a way to make it easy for web developers to let Google speak your answers out loud. From Google:
“The speakable schema.org property identifies sections within an article or webpage that are best suited for audio playback using text-to-speech (TTS). Adding markup allows search engines and other applications to identify content to read aloud on Google Assistant-enabled devices using TTS. Webpages with speakable structured data can use the Google Assistant to distribute the content through new channels and reach a wider base of users.”
So it basically works like this: if you want to throw your hat in the ring for “speakable answers” in the SERPs, put this markup in your site. Unlike other schema, make sure the data is in the title tag / meta description. You can also check out Google’s specifications.