When your iDevice is locked and you receive a call, text message, Whatsapp, Facebook update or most other kinds of updates, you will usually receive a notification on your lock screen (depending on your Notification settings of course).
There's a super little shortcut to allow you to go directly to the item you want, instead of having to slide to unlock, then enter your passcode (if relevant), then find the app you need in order to view the message.
Simply swipe the item itself, i.e. swipe on the message instead of the "swipe here to unlock" bar at the bottom. Swiping the message or item you want will have the same effect as swiping on the bar at the bottom, but it will take you directly to that message or item once the phone is unlocked!