In WordPress, trackbacks lets you notify a participating external blog that you have linked to their post.

As a practical matter, trackbacks is the name of the field in the post editing screen where you enter the url of the external blog post where you want your trackback notification sent.

To Trackback, or NOT to Trackback?

In general, this SEO blog recommends against the use of trackbacks. They are more trouble then they are worth. If you like a site, then link to it the old fashion way. Use of trackbacks, simply begs for unscrupulous people to scrap your site. Quite a few blogs have stopped using trackbacks, for a very good reason. They don’t work anymore, if they ever did.