A feature of the blogging software Movable Type. When popular bloggers make an interesting post it is likely to be linked to and discussed elsewhere in the blogosphere. The 'trackback' link leads to a page showing links to these posts and portions of their content.
A bit like e2's softlinks, this is a useful mechanism for finding out 'how did everyone get here?'. It's an indicator of one's blogspace neighbourhood (see also blogroll). The feature
works better than simple HTTP 'referer' links (backlinks/linkbacks) for following conversations, because it explicitly keeps track of both the two posts. Also, it doesn't rely on anyone actually clicking the link to set up the entry. However, it means that the linking blogger must send a trackback ping for each link they want to be trackable.