At times, you may see the above message in place of what you were sent from a contact. Due to end-to-end encryption, you may need to wait for someone's message to arrive because their phone needs to come online and properly encrypt the message for you. This can typically occur if you or the person you're chatting with recently reinstalled WhatsApp.
To speed up this process, ask the person you're messaging to open WhatsApp on their phone.
For more information about End-to-End Encryption, please read this article.
WhatsApp Support Team