This filter lights the image with a soft glow, like the old trick smearing vaseline on the lens. Softglow (legacy) produces this effect by making bright areas of the image brighter.
Your changes are displayed in this preview before being applied to your image.
The glow radius parameter controls the sharpness of the effect, giving a "vaseline-on-the-lens" effect.
The brightness parameter controls the degree of intensification applied to image highlights.
The sharpness parameter controls how defined or alternatively diffuse the glow effect should be.