When you’ve added a comment to a website, it would be nice if Google picked up on that immediately. That’s not always the case, however, and so it makes sense to let Google know. You do this by “pinging” it, to say there’s something new here. Some people don’t think this is worth doing, but I think it takes so little effort that it’s worth a try. Just use a ping service such as Pingomatic or Feed Shark to let Google know.