When you are on the actual course page, at the top there should be a bit that looks like this:
DISCUSSION (8235) Leave the discussion
Everyone Following Most liked Your comments
If you click everyone, that shows all the discussions.
Following just shows the people you are following and is perhaps the easiest to look at to start with.
Most liked shows the ones that other people have ticked to say they like or agree with.
Your comments, what you have posted and their replies.