a QR code (you need to scan it and follow the instructions featured in WhatsApp Desktop's main interface). Once that's done, all conversations and contacts on your WhatsApp will be synced so you can talk to them from your Mac just like you would on your smartphone.
With this tool, you can send multimedia content, create and manage groups, save conversations, make changes to your profile, and even take photos with your computer's built-in camera. That said, to run this app you need to make sure your smartphone is synced to the app, which is why the device needs to be on and share the same web connection to enjoy.