This Security feature was introduced first for iOS users in October 2018 to protect
chats from being seen by others who use the same device.
Currently this security feature is in the beta stage of development and is currently disabled by default. The feature will reportedly be available within the app under a new section in the Settings > Account > Privacy
How Fingerprint Authentication Feature will work for Whatsapp:
Once the fingerprint authentication feature is enabled, it will act as a blanket feature that will hide particular conversations or contacts behind a biometric wall.
If the app fails to recognise the fingerprint, users will be able to launch the app using old style PIN Lock.