WhatsApp has made an announcement
that its app can now be unlocked with a fingerprint on Android. You will need to use your fingerprint to access the app even after your phone is unlocked. WhatsApp claims that it is just an extra level of security, banking apps use a similar system.
The update is not available in all regions currently but once it arrives, you will need to turn on the function from the Privacy menu within the app. You can leave the app unlocked for 1 or 30 minutes, moreover you can prevent contents of your messages from appearing in notifications.
The feature was not a surprise. It was mentioned since WABetaInfo reported it in a beta version of the app back in August. Android is limited to just fingerprint unlock but with iOS face unlock is also an option.