Shadow squadrons are additional squadron numbers allocated to RAF training, Operational conversion, and Operational and Weapons evaluation units. Normally in peacetime these units are not tasked with combat roles, however UK planning for a major confrontation during the Cold War would have seen the tasking of the aircraft and pilots of these units in combat roles with the activation of the shadow designations.
Shadow squadron activation would have seen for example the RAF's Hawk trainers flown by flight instructors and pilots from the Red Arrows in the point air defence role with guns and sidewinder missiles.
With defence cuts and the disbanding of historic squadrons the "double number plating" of squadrons was a means of preserving the histories and lineages of such squadrons against the day that they could be revived.
The original article can be found at Shadow squadron and the edit history here.