Noise is the digital equivalent of grain in the film world. It is formed by longer exposures from longer than around 1/15 second and when the sensitivity of the sensor is increased to provide equivalent 'film speeds' above 100 ISO. It is more noticeable in small sized sensors[2/3inch] than the larger versions found in DSLRs with larger and 'full size' sensors.
A
Original shot at 800ISO which is comparatively noise free compared to film, but can be improved.
B
B -- has been treated at the default level of the Helicon programme and this softens the image as it merges noise and surrounding area.
So one needs to regain sharpness with USM or other technique. I sharpen just the luminance channel after splitting image into Hue, Saturation and Lightness channels.
C