in the short sense its an effecting using a raster image technique. A raster image is the same thing as a bit map. Each byte in the image file represents the visual out come. So no compression or transparency, just raw pixels stored in a file. Now a raster effect is an effect using the pixels not the video cards magic. So if you want to add a shadow to a circle in raster you do so by manipulating the pixels around one or two sides of the circle. Today, and for quite some time, most effects are layers on top or behind an image. Lots of fancy stuff out there to make really neat effects. Raster effects are doing it on the image, effecting the data that is already on the screen, no tricks.
now to put it to emulation, I think what you are reading is how mame can raster an image to make it look like a real monitor? An example of that is to skip ever other line ( scan lines ). I could be offer here but I think that is what your asking. Yes an arcade monitor produces what we call scan lines and to make it look like that on a computer we raster the image to produce that effect.