What is Emissivity?
According to the Merriam Webster dictionary, emissivity is
: the relative power of a surface to emit heat by radiation
: the ratio of the radiant energy emitted by a surface to that emitted by a black body at the same temperature
So, you could think of it this way: every material or object emits radiant heat or infrared radiation at its own specific rate. The specific rate at which it emits thermal energy depends on the type of material it is made of, its surface properties, and the temperature of the object itself.