What is emissivity?
Emissivity describes an objects ability to radiate energy.
Some objects have a high emissivity, these are typically non metals. Other objects have a low emissivity, these are generally metallic. Emissivity of a material is a value between 0 and 1. A blackbody would have an emissivity of 1, this means that it emits 100% efficiently. A perfect mirror would have an emissivity of 0, this means that it emits none of its own energy, it just reflects 100%.
The thermal image above shows a heated piece of metal with PVC tape at each end. The PVC has an emissivity of 0.95, which means that it is emitting 95% efficiently. The metal has an emissivity below 0.1 which means that it is emitting less than 10% of its own energy.