Siri is an intelligent personal assistant built into iOS. You can ask Siri to send WhatsApp messages or place WhatsApp calls, and Siri can also read aloud your unread WhatsApp messages.
Note: These features are only available on iOS 10.3 and later.
For specific information on how to use Siri, read this Apple FAQ article.
Note: Opening WhatsApp with unread messages will reset the notification badge. Siri will then interpret this as if there were no unread messages and will therefore be unable to read them aloud.
For information on Siri and how Apple handles data sent through Siri, read this Apple FAQ article.