A deinterlacer is a type of video card that was designed to make using a VGA type monitor look better when using the Amiga. It has a standard VGA type connector, however it does not pass through VGA video displays, its purpose is to stop the flicker from interlaced displays. Many of these cards were supplied with software for OS 1.3 which allowed you have  variable sceenmodes. This was only a patch however. It has been rumored that they work OK under OS 2 providing a flicker free display in 640*400 60Hz (NTSC)