High Pass is similar to the frequency filters used in electronics. It eliminates low frequencies while keeping high frequencies. The result is a final image where high-contrast areas are kept as-is, while the rest is replaced with 50% gray. How this based effect is harnessed depends on the Mode setting.
Radius sets to what extent this filter is applied to nearby pixels. Raise this value to make contours more visible. The best value to use here depends on the picture’s resolution.
Switch the Mode setting to Overlay to increase the picture’s local contrast in dependence on the radius setting.
The Soft Light mode emphasizes details in the picture.
The Normal mode simply displays the high-pass output, where low-contrast areas are replaced with 50% gray.
Use the last of the settings here to set whether or not to Preserve edges and the Intensity for this. In Preserve edges mode, the transitions at edges in the image have less contrast, and contours at these transitions are suppressed.