It is the amount of carbon stored in the organic fraction of soil, measured as stock in the first 30 centimeters and expressed in tons per hectare.
It matters because organic matter is the engine of almost everything good about soil: it retains water and nutrients, supports the structure, feeds microbial life, and makes soil endure tillage without degrading. A soil with good carbon forgives handling errors; a poor one, no.
The country's median is 5.2 t C/ha. Values above usually correspond to the humid pampas and soils that were under sown pasture for a long time; the low ones, to soils sandy, to arid areas or to paddocks with many years of continuous agriculture without replacement.
It is a 1 km raster, that is, a regional value. It is used to compare areas, not to decide fertilization: that requires analysis of soil of paddock.