It's a string of characters - a mixture of arabic text and unicode - that is able to shut down iPhones instantly.
Don't worry too much if you get it, apparently all you need to do is reply to the message to undo it.
You can also try:
Click SETTINGS -- NOTIFICATIONS -- MESSAGES
Turn OFF 'Show Onlocal Screen'
Choose NONE for 'Alert Style When Unlocked'
Apple has confirmed the existence of the bug: “We are aware of an iMessage issue caused by a specific series of unicode characters and we will make a fix available in a software update,” an Apple rep told to Reuters.
This is what happens: