It's a sender unit that fits in between your speedo cable and your gearbox and sends a signal to your fuel/trip computer telling it how fast you are going.
If you dont have the trip computer then there is a good chance you dont have one, but they can be picked up for pennies if you do want one to use with the AVC-R!

Looks similair to this but a little fatter
http://cgi.ebay.co.uk/Ford-Escort-mk...2em118Q2el1247