This feature will allow users to set a PIN code on the app requiring you to enter the code each time you open up the app.
Here's how to set a PIN (or Passcode) lock to prevent unwanted people opening the app and viewing your posts:
iPhone users: From your profile page, select 'Edit profile' from the settings (cog) menu. The 'Set passcode lock' option is where you set it up.
Android users: From your profile page, select 'Settings' and simply enter your preferred PIN number under the 'PIN Lock' section to secure the app.