Emission of an electronic product is the unwanted electromagnetic energy released to the environment in the form of radiated and or conducted noise.
Emission is categorized as
Radiated Emission (RE)
Unwanted noise radiated out from you product in the form of electromagnetic radiation.
Conducted Emission (CE)
Unwanted noise conducted out from your product to the power cord in the form of voltage waveform.
Summary & Conclusion
Emission are the source of electromagnetic noise.
Emission is common source of electromagnetic interferece (EMI) that degrades wireless performance of each connected device. Regulatory agencies such as FCC and CE has strict requirements of emission before any products can be shipped in NA and EU.