You can add a new message to a conversation or group chat that you are in.
This is a convenient way to mark message text as high priority and ensure that it is acknowledged and not just read.
When you have tapped this icon, the message text that you are typing appears in red on your screen.
The sent message appears on the screen of all other participants in the conversation or group chat. If you have sent a message that requires an acknowledgement, the screen indicates this:
If the message has been acknowledged by all participants, you see this:
For a complete list of all possible statuses for sent and received messages, see Message States.