This filter detects the edges in a filter. For each pixel, it looks a each channel, finds the local gradient and replaces the channel by a value determined by the gradient. Edges become white while flat areas become black. You can choose between various kernels for calculating the gradient.
You can use this filter to obtain black edges by using the Grayscale and Invert filters on the results. This can look nice when overlaid on the original image with the Multiply blend mode.