This is a feature to use Touch ID, a finger scan sensor, instead of a passcode you set when you accessed to the app or locked folder.
In order to use this feature, you should set a Fingerprint in the iOS Settings > Touch ID & Passcode.
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Tap Settings at the bottom and select General.

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Activate Touch ID.

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To access to the app or locked folder, Tap the Home button lightly with your finger and unlock.