“Slow shutter” is a video term for an electronic shutter that is open for longer than the duration of a single recorded frame. It’s not actually a shutter as in a stills cameras physical shutter, most video camera don’t have a physical shutter. Normally a video camera operates at 25 or 30 frames per second. So, the camera sensor normally captures light for 1/25th or 1/30th of a second in progressive, for interlace it’s half of this as the shutter is open for the duration of each field, one field is half the duration of one frame, before writing the data to one frame of the video.
The sensor is then reset, captures for the next 1/25th or 1/30th and then writes the next frame and so on, creating a video sequence. With a slow shutter the sensor is allowed to capture light for – (the slow shutter speed in fames) x (the number of frames) before that data is written as a single frame. So with a 16 frame slow shutter the sensor is allowed to capture light for 16 x 30 (or 25) frames before creating an image.
So at 30fps, one frame lasts 1/30th. Therfore, 16 x 1/30th = 0.53 seconds. The sensor is being allowed to capture light for 0.53 seconds before getting reset. If you do not use an interval recording mode or time-lapse, each of the 16 video frames the camera records while the sensor is being allowed to capture light gets written with the same image data (from the previous shutter cycle). So with a conventional video recording the image only refreshes once every 16 frames.