The Noise Reduction tool allows you to reduce "noise"
that is often present in digital photos as a result
of the sensors in digital cameras.
The tool reduces noise in either color or grain (or both).
The initial settings for thr noise reduction tool are good for reducing the noise introduced by the sensors in digital cameras. If additional noise is present, for example as the result of a high ISO setting being used, then you may need to use the sliders to reduce noise further.
Note: For raw photos, you should use the Color Noise slider in the Raw Adjustments tool first. If you want further noise reduction on only part of your photo, then you can use the Noise Reduction tool with a mask.
Note: Noise, and reductions to it, are most easily seen while looking at a digital photo at the pixel level by selecting View > Actual Pixels.