Plain English Meaning
On some approaches, if you have to go around, you must climb more steeply than usual to stay clear of terrain or obstacles. The chart tells you exactly how steep and up to what altitude.
Full Definition
A required climb performance, published on certain instrument approach procedures, that exceeds the standard 200 feet per nautical mile climb gradient used for missed approaches and departures. When a higher gradient is needed to clear obstacles or terrain along the missed approach path, the chart specifies the required feet per nautical mile and the altitude up to which it must be maintained.