By default, as you read new discussion posts, Canvas will mark them as read (changing the indicators from blue to white) as you scroll down the page. However, you can manually mark each posts back to a read or unread state.
You can tell Canvas not to automatically mark all your posts as read in your Discussion settings.
Note: Once a post's state is manually changed, the post will not change states (become read or unread) until you manually change it again.