Edit a list by swiping left on a shopping list row and tap on "Pencil" icon to open the Edit screen. Under "PASSWORD PROTECT" section, tap on "Set Password". This will bring up a screen to enter a pincode. Once you enter a pincode and save the list, the next time when you tap on a list you will receive a dialog to enter this pincode.
For your convenience you could also use your finger print to validate your pin.