What is a Trackback?

In short a trackback is when site A links to site B and in turn site B links back to site A.  (more or less).  You do not have to link to the sites from your site for this to work, scrapebox sends trackbacks automatically and fakes it like you are linking to their site.  So that they will link to your site without you having to link to theirs.

For a technical definition you can read more on trackbacks here.