A fingerprint is utilized to unlock the device. For the fingerprint scanner to operate, hands must be clean and free of moisture, dirt, oil, lotion, dyes, etc.
Up to 3 fingerprints can be registered on the device.
From a Home screen, navigate: Apps > Settings > Fingerprint.
Tap Fingerprint manager.
From the "Register fingerprint" screen, swipe your finger 8 times down the pad located at the bottom (as shown in the on-screen tutorial).
Swipe entire finger pad from base to tip of finger across the home key.
Keep finger flat against the home key and swipe slowly.
Indicators turn green when successful.