We use fingerprint verification for added security. To confirm your order, simply place your finger on your phone's home button. Hold it there for a moment, but do not press.
On your phone, if you go into Settings, then Touch ID & Passcode, you should be able to see that you have a fingerprint setup. You can also set up another one there if you'd like.
The passcode the app is requesting is the same 4 or 6 digit passcode used to unlock your phone screen.