This filter, accessible through Filter/Blur/Smart Blur, works to remove low-intensity "noise" from an image. It does this by preserving the areas where there is a sharp color contrast, such as edges, and blurring the areas where the colors are almost the same (within a tolerance set by you). As you can imagine, this is useful when treating grainy photographic images, since the grainy areas are usually only slightly different in color from the surrounding areas. The real detail of the image is largely left untouched.
In the "Smart Blur" dialog set the following:
Use the slider to set the Radius - or type it directly into the box - between 1 and 50. The radius is a measure of the distance from the center of each pixel and determines the extent of the color averaging that constitutes the blurring process. Naturally the higher the radius, the greater the blurring.
Set the Maximum Delta, again by using the slider or typing, between 0 and 255. This is a measure of the threshold for blurring. At low values there is blurring only between very similar colors; at high values almost any colors, no matter how dissimilar, may be blurred together.
The effects of your filter settings are displayed in the Preview box.
See: