Emissivity is the measure of an object's ability to emit infrared energy. Depending on the color and/or reflectivity of the material you are measuring the temperature of, the temperature gun needs to be calibrated to that material or you will get a false reading. Emissivity is generally as low as zero for an extremely shiny surface to as high as 1.0 for a surface painted matte black.

Unless the temperature gun has been calibrated for the emissivity of the material you are measuring the temperature of, the reading will be incorrect.

I understand some of the cheap IR temperature guns don't even have the ability to calibrate to the emissivity of different materials which IMHO renders them next to useless.