Subcooling is a technical expression for a refrigerant in liquid phase at a temperature lower than the corresponding evaporating temperature to the actual pressure.
Evaporation and gas phase
Like water at 1 bar below 100°C is steam (gas) all refrigerants have a similar liquid, evaporation and gas phase.
The boiling temperature for a refrigerant is pressure dependent, at a curtain pressure relates to a specific boiling temperature, for example R717 ammonia is a 2.9 bar boiling at -10°C and 11.7 bar boiling at 30°C.
If for example ammonia is 25°C at 11.7 bar it is subcooled 5°C below boiling a temperature 30°C.
The advantage of subcooling
In a cooling plant there is an advantage in having the liquid subcooled. If the liquid line is exposed the a heat flux there is a risk that the liquid will start to expand before the thermal expansion valve, flash gas, and the evaporator will fail to work. Subcooled liquid, however, can accumulate energy reducing the risk for flash gas.