You'll find option in the settings menu; when you turn it on, Whatsapp asks you to create a six-digit passcode that you'll need to enter every time you try and log in to the service on a new device. You'll also need to add an email address that you can use to reset your passcode in the event that you forget it.
Once that's set up, the phone number associated with your Whatsapp account will be protected. Since Whatsapp logins are tied to a phone number rather than an email address, the usual method of texting a randomly generated authentication code to your phone won't work here -- hence the user-generated passcode. Either way, it's good for the service to finally add a way to keep accounts more secure.