Discussion as an assignment
Your teacher can set a discussion as an assignment to clarify that it's mandatory to participate in.
If a discussion is set to be an assignment you will find it in either the ToDo list, on the course homepage or in the Assignments list. In the list of discussion topics there will be a small assignment symbol in front of the topic, marking it mandatory.
When you enter the discussion there will be a text "This is a graded discussion" to notify you that the discussion is set as a graded assignment.