Lower values preserve more detail, but aren't as smooth. Strength: Controls how different the area around a neighbor pixel can look compared to the center pixel before it's no longer used for denoising.Higher values are smoother, but might lose detail and are slower. Radius: Size of the image area that's used to denoise a pixel.
The default settings fit a wide range of scenes, but the user can tweak individual settings to control the trade-off between a noise-free image, image details, and calculation time. On rendering, it will denoise tile by tile once all the surrounding tiles are finished rendering. To use the option, enable it in the render layers tab of the properties editor. Denoising filters the resulting image using information (known as feature passes) gathered during rendering to get rid of noise while preserving visual detail as well as possible.
