I am not sure where you have seen this expression, but it is usually used to mean movement past the point required for normal operation.
Example, if you have some kind of switch, you push the operating lever a distance of 10mm and the switch operates, if you then move the lever a further 5mm before reaching the mechanical stop, you have 5mm of overtravel.
Google does hit something, but it takes a little bit to interpret it - Two links: http://www.nrc.gov/reading-rm/doc-collections/gen-comm/bulletins/1979/bl79025.html **broken link removed**
Overtravel seems to be related to induction disk overcurrent relays. The second link has some diagrams. The first link specifies overtravel in mils - insufficient overtravel means contacts don't close properly. There were some other links that seemed to indicate that overtravel also causes a time delay between the relay's deactivation, and when the contacts actually open.