Teachers and students may send and receive messages via Interact. This is in addition to receiving notifications about assignments, forum discussions etc.
New messages are highlighted with a number in the messaging menu at the top of the screen:
The number reflects the number of people who have sent new messages, and not the actual number of new messages.
Clicking there displays the new messages, along with the option to send a new message, to mark all as read, to go to your preferences page to change how messages are received and to view all messages.