Superheat is the temperature of vaporized refrigerant above its saturation temperature. Depending on the pressure in the system, the refrigerant condenses from a vapor to a liquid at a certain temperature. Refer to a PT chart for exact pressure to psi dew and saturation point. Every degree above the refrigerant's saturation point is 1 degree of superheat.
As refrigerant pumps through your system, it is constantly changing states. From a liquid to a vapor and back and forth. As the refrigerant passes through the indoor evaporator coil it picks up heat. Ideally, the refrigerant should leave the evaporator with 10 degrees of superheat. It is the metering devices job to insure that the right amount of refrigerant sometimes referred to as "Freon" is allowed into the evaporator. Having the right amount of refrigerant in the evaporator is vital to ensure that just the right amount of heat is absorbed. Superheat is important because after the refrigerant leaves the evaporator it travels to the compressor. The compressor will fail if liquid refrigerant is pumped through it. The liquid will flush away all the oil from the compressor eventually causing complete and utter compressor failure. If refrigerant leaves the evaporator with 10 degrees of superheat as it travels back to the compressor it is picking up more superheat. That ensures that the refrigerant is completely vaporized when it reaches the compressor, giving you a long life for your air conditioning compressor.