Inside the evaporator, refrigerant boils at a temperature fixed by its pressure. Once the last droplet has evaporated, any further heat raises the vapour's temperature. That extra rise is superheat. A technician measures it by reading the suction pressure, converting it to a saturation temperature and subtracting that from the actual pipe temperature at the evaporator outlet or compressor inlet.
Superheat tells you how well the evaporator is fed. High superheat means the coil is short of refrigerant, from a low charge, a blocked capillary or a starving valve, so part of the coil is doing no useful cooling. Very low or zero superheat means liquid may be leaving the evaporator and heading for the compressor, which can wash out oil and break valves.
For expansion-valve systems, the valve is set to hold a modest superheat, typically a few kelvin, chosen by the designer. For capillary systems the superheat floats with conditions but is still a key diagnostic number. It is one of the first readings a competent technician takes when a cabinet is not cooling properly.