Within each assignment and on each screen you will notice a speech bubble in the right-bottom corner of your screen.
By clicking the speech bubble, the activity overlay will appear.
The activity overlay will show you all the activities related to the 'module' you have opened the overlay in.
So if you open it in Finances, it will show all the activities for Finances, if you open it in Documents it will show all the activities for Documents and so on.
You can make a note by entering the note in the message box and not mentioning anyone.
A message can be send by typing in your message in the message box and selecting a recipient via the @-mention button and select the recipient.