This key contains the original ID of road sections as imported from the AND Data. These IDs were originally intended to be used for updates or the "future" import of other bits of that dataset like house numbers. Both were never done and OpenStreetMap no longer has access to the bits that were not imported at that time. In the intervening years a lot of roads have been merged and/or split, so these IDs are no longer accurate anyway.
Feel free to remove this tag when you encounter it when mapping. Like with all other tags that became really obsolete (like created_by) don't start specifically deleting them, as that causes unnecessary bloat of the OpenStreetMap main database. (A delete creates a new version in the history too.)