Scan lines are a series of parallel lines that were sometimes visible on older CRT television and computer screens.
This effect is is often employed as a graphic design device to add interest to an image. This time I left the Enhance slider at zero and clicked the tickboxes next to Feedback and Scan color to turn those individual effects off. You can experiment with different settings on the Raster intens, Raster scale and Raster width sliders until you produce an effect that you're happy with.