I get this question a lot, ‘what is a trackback?’  Wikipedia says, ‘a TrackBack is a mechanism used in a blog to show, around an entry, a list of other blogs that refer to it.’  They continue:

The term TrackBack was introduced by Six Apart which introduced a mechanism in their blogging server, Movable Type, that works by sending a ‘ping‘ between the blogs, and therefore providing the alert. The blog receiving the ping typically displays the TrackBack information below a blog entry. This usually includes a summary of what has been written on the target blog, together with a URL and the name of the blog. The Referer field in the HTTP protocol was originally intended as a means of supporting features similar to those TrackBack offers.

Tom Coates has answered the question here.  Another beginner’s guide can be found here.  The official specification can be found here.  Trackback issues relative to WordPress can be found here.  Good luck!