A trackback is basically a way for other blogs to cite your blog entries. You can find more information about them <a href="http://www.google.com/search?q=define:TrackBack&sa=X&oi=glossary_definition&ct=title" target="_blank">here</a>.<br /><br />
Please note sometimes these are used for spamming purposes. The settings lower on this page help reduce any possible trackback spam.