Comments allow users to interact with the content on a site, to respond to an article, offer their own ideas, make additions, or supply a critique.
When you bring up an article to read, look for comment-related links at the bottom of the article. If you're not logged in, this might read "login or register to post comments". When you do log in, you should see something like "Add new comment". Click on the link and you're ready to comment away.
Comments can be a great way of enriching a community site--but they can also lead to unfriendly, even harassing exchanges. As with any communication, it's important to try to ensure that your comments are respectful and constructive.
Comments on a Drupal web-site are "threaded". This means you can comment directly on an article--or you can reply to an existing comment. If you reply, your comment will be indented to show that it is part of that discussion.