Many publications including The Guardian reported that the messaging app WhatsApp was insecure and hackable. The creator of that encryption protocol, Moxie Marlinspike from Open Whisper Systems, posted on his blog that this was incorrect. Now a large number of security professionals have written an open letter to The Guardian asking them to retract the story. There is no back door in WhatsApp, and the article was wrong. It was written in a sensational way to drive traffic.
If you use WhatsApp, you should know that it is secure, but there is a setting you can turn on to make it even more secure. You want to turn on the notification of when recipient's key changes, so that you know before your message goes through to them. That will prevent someone from hijacking someone's account, changing the keys, and still getting messages from them.