Superheat
Superheat is how many degrees the refrigerant vapor at the compressor is above its boiling point at the current pressure. It tells you whether all the liquid boiled off in the evaporator (good) or some liquid is coming back to the compressor (bad — liquid slugging destroys compressors).
Subcooling
Subcooling is how many degrees the refrigerant liquid at the condenser outlet is below its condensing point. On TXV/EEV systems, subcooling is the primary charge indicator.
Why homeowners should care
Charging by gauge pressures alone is guessing. Ask for superheat and subcooling numbers. They should be on your invoice.