WAbetainfo shared a screenshot in which you can see how this new feature works on WhatsApp Desktop beta when it is available in the near future.
You can respond to messages using one of six different emojis, as shown above. The reaction button is a new addition.
The reaction button is not always visible: a new reaction button appears when the finger is near a chat or group message.
You will be able to respond to the message by pressing the reaction button. The new feature is still in development, but as WhatsApp is working on it