Speakable (schema.org)
Speakable is a schema.org property that uses CSS selectors to mark individual sections of a page as suitable for speech output, originally intended for news content read aloud by smart speakers.
In practice
Google explicitly labels Speakable a beta feature in its documentation: requirements and guidelines may still change, and use is limited to news content in a few languages and regions. Support from search engines and assistants is correspondingly narrow.
For most websites Speakable is therefore better treated as something to watch than as a priority investment. If you have to prioritise, start with more broadly supported types such as Organization or Article. One side effect still makes the markup worthwhile: it forces you to name the one paragraph that answers the page's core question on its own – and that pays off regardless of any voice output.