This property controls whether this light only affects lightmap baking, or dynamic objects, or both.
If this setting is LightmappingMode.Realtime, this light only affects dynamic objects, and it will not have any effect on baked lightmaps. If this setting is LightmappingMode.Baked, this light only affects the baked lightmaps, but not dynamic objects. If this setting is LightmappingMode.Mixed, this light affects both baked lightmaps and dynamic objects.