What are Trackbacks?

When using the advanced reply, what does, Send Trackbacks to (Separate multiple URLs with spaces) mean/do?
A 'trackback' is a link to another web page. It comes with a 'pingback' so that the link is reciprocated.

I have used one here so that we can see what it does.
I think Bad CO has been doing a bit of tidying up in order to allow people to see the answers to questions about the new format.

Anyway, I put a trackback link to the last post of the lalalalalalalala thread. I can't see any sign of it on here. I can't see any sign of it when I go to 'Edit' on my post above, I can't see any sign of it on your lalalalalalalalalalast post. I think it is either
a) A feature which is unnecessary, or
b) broked.
Why do I permanently have my message notification light lit telling me I have three unread messages? I have marked them as read but to no avail.
Suicidal of Staffordshire.