The little letters beside your messages are receipts giving you more information about the status of your message.
If you see these characters instead:
... Three dots mean that the app is trying to establish a connection. Your message will change to S as soon as a connection is established. If you see the three dots for an extended period of time, check your data and/or Wi-Fi connection.
! Red exclamation mark means there is an error sending your message. In this case, you'll need to send your message again.
A faded D appears next to messages sent to iPhones and Windows Phones. It means that we've sent the push notification of your message to your friend but they haven't opened the app to receive the message on their phone. Once they open the app, you’ll see a solid D.