Up to now you had to write a comment on a post in order to be kept up to date on any developments in that topic. Now you can choose to follow a post without commenting; just tap the open star icon under the post to start following. The star will turn blue, and you will be notified of any future comments to that post. If you like a post or a comment (by tapping the thumbs up icon), or write a comment of your own, you will automatically start following the post.
To unfollow, simply tap the star again. This will not remove your 'like' or your comment if you have made one, but you will not receive any further notifications about others' comments.