That means that you and your instructor do NOT have to be logged in to the system at the same time. You can log in whenever you like and post a question for your instructor. Other students can then see your question. The newest question is always placed at the top of the list. Instead, you must contact him or her in person or by email.
Many instructors have their email addresses available on their homepages.
Some students like to browse through all the questions and answers regularly. You can learn a lot from the questions that other students have asked. You will see a list of announcements with the newest appearing at the top of the list. For each of your classes, there is a Class Discussion page. Here, the instructor or any enrolled student can start a new discussion topic or contribute to an existing one.