At times, you might see the above message in place of what you were sent from a contact. Due to end-to-end encryption, you might 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, you can try asking the person you're messaging to open WhatsApp on their phone.
For more information on end-to-end encryption, please read our white paper and this article.