An Ur-box was a complex device of Time Lord design. It recorded events of a specific point in the timeline. It projected that data and information into the Time Vortex, sending false information to time scanners of time-active factions. This allowed the users of the Ur-box to operate without being detected and so change a point in the timeline.
There were, however, several problems with the technology. It had a limited sphere of influence which necessitated that there would have to be no excursions outside the field. Events within the field needed to be match those of the Ur-text and so individuals were required to act out events from the "real" timeline.