From Wikipedia: A Trackback is one of three types of Linkbacks, methods for Web authors to request notification when somebody links to one of their documents. This enables authors to keep track of who is linking to, or referring to their articles. Some weblog software programs, such as Wordpress, Movable Type and Community Server, support automatic pingbacks where all the links in a published article can be pinged when the article is published. The term is used colloquially for any kind of Linkback.

From me: It is used a lot on blogs when folks link back and forth to each other. It helps with Search Engine Optimization (gives you higher rankings on search engines like Google or Yahoo!) Probably nothing most of us have to worry about, but of great interest to bloggers, web designers and owners of forums like this.

Hope this helps!