X marks the spot. In the comment stream, within each comment in the upper-right corner you will see an [X] indicating your ability to bookmark the comment as your "already read" point.
When you do this, that comment and all older ones will be hidden from the view. (The bookmark is saved as a browser cookie, so this is only remembered within the browser on your end, and your browser must be accepting cookies.) All comments above the bookmarked point will remain visible to you. So if you bookmark the most recent comment (the one at the top), that means all comments are considered to have been read and you will see no comments until new ones are posted.
You can clear the bookmark at any time and reveal all comments by clicking the "clear bookmark" link at the top of the comment stream.
Note: The bookmarked point does not apply to filtered searches within the comment stream. Whenever you use the search feature, all results, including those older than the bookmarked item, will display until you clear the filter.