This attribute gives the appearance that the material is emitting incandescent light.
Note that a Mesh Light may work better in a situation where you need an object to emit light that casts realistic ray-traced shadows.
Controls the amount of emitted light. It can create noise, especially if the source of indirect illumination is very small (e.g. light bulb geometry).
Bulb with emissive Standard Surface shader assigned to it (rollover image)
Increasing the number of Diffuse samples will help to reduce any noise in dark, indirectly lit areas of a scene when using emission.
The emitted light color.
Texture map representing hot lava connected to Emission Color