script-fu-simple-contrast-mask adds a contrast mask layer above the current active layer. A contrast mask brightens the dark parts of an image and darkens the bright parts. This serves to enhance details.
The script is called "simple" because it does not ask any questions. It assumes defaults for all parameters. Specifically, the radius for the Gaussian blur is set to one percent of the shortest dimension of the image (width or height). The user can adjust the blending level after creation by adjusting the opacity in the layers dialog.
To use the script, copy the script-fu-simple-contrast-mask.scm file into the scripts folder under the .gimp-2.6 folder in your home folder. Then click on the Filters menu in the GIMP's main window, go down to the Script-Fu item, and select "Refresh Scripts". You should find "Simple Contrast Mask" at the bottom of the Colors menu.