If Passbook notices you walk past a venue for which you have a pass, or if a timed event like a flight or concert is coming up, Passbook will present a Local Notification with the relevant pass for quick access.
If you have a passcode set on your device, you usually have to enter it when you slide the icon for most notifications like this, but not for Passbook. If you slide your pass’s icon, Passbook will display the pass without requiring your passcode. This makes it easy to quickly scan your affiliate card or tickets without having to enter passcode, but still prevent access to the rest of the device and any sensitive information.