Superheat is the extra heat in the refrigerant vapor after it has fully boiled in the evaporator; subcooling is the extra cooling in the liquid refrigerant after it has fully condensed. Together these two readings tell a technician exactly how a system is charged and how each coil is performing — they are the difference between charging by measurement and charging by guesswork.
The correct method depends on the metering device. Modern systems with a thermostatic expansion valve (TXV) are charged to a manufacturer subcooling target, usually about 10 to 15°F. Older fixed-orifice systems are charged by superheat, typically in the 10 to 20°F range read from a chart. High superheat with low subcooling means the system is undercharged (often a leak); excessive subcooling means it is overcharged.
Why it matters to you: a system charged 'by feel' until the line is cold can easily be left over- or under-charged, which quietly raises your energy bill, weakens cooling, and shortens equipment life. Proper superheat in particular protects the compressor by ensuring only vapor — never damaging liquid — returns to it. A tech who hooks up gauges and references superheat and subcooling (and weighs in refrigerant) is doing the job right; 'I added some until it felt good' is not a charging method.