Here's why:

* Emissivity is the ratio of the energy radiated by a material to the energy radiated by a blackbody at the same temperature. A blackbody is a theoretical object that absorbs all radiation incident upon it and emits radiation perfectly.

* Blackbodies have an emissivity of 1. Real materials have emissivities less than 1, indicating they don't radiate as perfectly as a blackbody.

* Emissivity depends on the material's composition, surface finish, and temperature. Different materials have different atomic structures and electron configurations, influencing their ability to absorb and emit energy.

Examples:

* Polished metal surfaces have low emissivity, meaning they are poor emitters of infrared radiation.

* Rough, black surfaces have high emissivity, meaning they are good emitters of infrared radiation.

Therefore, emissivity is a fundamental property of a material that affects its ability to radiate energy.