Plain English Meaning
A lookup chart that tells you how much extra height to add to your altimeter reading when it is very cold outside, because cold air makes the altimeter show you higher than you actually are.
Full Definition
A reference table published by the International Civil Aviation Organization that shows how many feet to add to a published altitude to correct for altimeter under-reading caused by colder-than-standard air. The table is organized by reported surface temperature and by height above the altimeter setting source, and is used by pilots to calculate a corrected indicated altitude that ensures actual obstacle clearance during instrument approaches in cold conditions.