Plain English Meaning
A special map controllers can pull up on their radar screen showing where the tall obstacles and high terrain are. They use it only in emergencies, when an aircraft has to fly lower than the normal safe altitudes ATC would otherwise assign.
Full Definition
An air traffic control radar display overlay used by controllers to assist an aircraft experiencing an emergency that requires vectoring below the minimum vectoring altitude (MVA) or minimum IFR altitude (MIA). The map depicts the location and elevation of significant obstructions in the controller's , allowing the controller to provide and information that keep the aircraft of terrain and obstacles when normal cannot be maintained.