Without sharpening, images appear soft and lacking in resolution. Experiment with moving the sliders and see how the changes affect the image. Amount determines the intensity of the sharpening that is done to the edges in the image. Radius determines the number of pixels to be adjusted on each side of the edges. A lower value sharpens only the edge pixels.
Threshold determines the range of values that define an edge.
If the range is set to 0 all pixels in the image are considered edge pixels and will have the filter applied. Then save your file.