You may enable passcode/password to protect your journals.

Go to Settings > Passcode > Enable Passcode

By default, 

  • Simple passcode of 4 digits is used. You can disable to use a complex password making up of alphanumerics.
  • Require passcode is set to immediately
  • Touch ID is enabled, if supported on your device

This passcode protection is different from your device's passcode.

It is a second authentication to further protect your journals.