A trackback is a system that allows a blogger to let another blogger know that she’s commented on one of his posts. This way, where two bloggers are discussing the same topic, if one refers to the other’s post in her blog, the other will be informed automatically. The signal sent to the blogger telling him he’s been cited in a blog is called a "ping". The ping takes the place of the customary message, “Hi, I’ve just commented on your post, on my blog".
This does assume, of course, that you’ve set up a trackback system on your blog, so non-geeks can stop reading now.