You can read any of the following links or articles to get a general understanding of a trackback or backlink. In short, they display other blogs or websites that "link" "back" to your post(s). This, generally is a good thing.
Typepad (from the Typepad Support pages):
• What is a trackback: http://tinyurl.com/59bdey
• Sending a trackback: http://tinyurl.com/6myz8u
• Acccepting a trackback: http://tinyurl.com/6gdhu3
Blogger (from Blogger Help pages):
• What are backlinks and how to use them: http://help.blogger.com/bin/answer.py?hl=en&answer=42533
WordPress (from FAQ pages and Forum find):
• How to send a Trackback: http://faq.wordpress.com/2006/05/11/how-do-i-send-a-trackback/
• What is a trackback: http://en.forums.wordpress.com/topic/what-is-a-trackback-1?replies=8
• Adding a trackback: http://en.forums.wordpress.com/topic/trackbacks-5?replies=10
• Overall search on "trackback" and all the results: http://faq.wordpress.com/?s=trackback
Hope this helps!