Emissivity is the fraction of infrared radiation a surface emits relative to what a perfect black body would emit at the same temperature. It runs from 0 to 1: matt paint is around 0.95, paper or plastic 0.90, and polished metal can drop to 0.05.
A thermal camera does not measure temperature, it measures radiation, and it needs the emissivity to convert one into the other. If the value set is higher than the real one, the camera underestimates the temperature; if lower, it overestimates. In electrical installations this is critical because bright metal surfaces are exactly what abounds.
Standard practice in electrical inspection is not to trust readings on bare copper or aluminium and to look instead for a reference point of known emissivity — matt tape, insulator porcelain, a painted enclosure — or to work by comparison between phases, which is immune to the problem because all three surfaces are alike.