Hi the plug-ins goal is to auto stack star trail images so you get a nice streak across the sky. The Light frames are added using lighten and dark frames using difference. The light frames are the shots of the sky with long exposure times of around 30s. Over this amount of time you can start to see the rotation of the earth, so the stars appears as short lines. Dark frames are taken with the same settings and in the same ambient conditions as the light but with the lens cap on so as to create an image of the sensor noise. By stacking 139 of them all you get something like:
The plugin first creates a black image so when you layer over the light frames with lighten you are starting from nothing as it where. The dark frames are then differenced to help reduce the noise and any hot pixels from the image.
If no images are found to layer you just get the black starter image. One of my todos is to count layers as they are added and then warn if none are found. What image extensions did you use as there's a white list of image types so I don't try open a non image file and crash out. The list is currently jpg, jpeg, tiff, tif, bmp and png.