If enabled in your class, your Reading List will include important posts in your class to help you stay up to date with class material. Some instructors might require you to read certain posts before you can post new questions. 

To view your Reading List, you can click on the Reading List folder located in your top bar:

To remove a post from your Reading List, click on the red I've read this button below the post Question History slider:

When creating a new post, you will be able to see if there are any unread posts in your Reading List:

If you would like to add a item back to your Reading List, you can click on the blue Add back to my reading list button: