In the condenser, hot vapour first cools to its condensing temperature, then turns into liquid at that temperature, and finally the liquid cools a little further. The amount it cools below the condensing temperature is the subcooling. It is measured by reading the discharge or liquid-line pressure, converting it to a saturation temperature, and subtracting the actual liquid-line temperature.
Subcooling indicates how much liquid is held in the condenser. Low subcooling can mean a refrigerant shortage, so the condenser does not fill with enough liquid. High subcooling can mean overcharge or a restriction downstream that is holding liquid back. Read together with superheat, it lets a technician separate a charge problem from a blockage.
Some subcooling is useful. Liquid that is a few degrees below its boiling point will not flash into vapour before it reaches the metering device, so the evaporator gets a steady feed. That is one reason a condenser needs clean fins and cool air: in a hot, poorly ventilated corner, subcooling falls and capacity drops with it.