Interlacing is the process used in
televisions to increase the apparent refresh rate of the screen. The early televisions in the
1950s were unable to refresh at high speeds. Instead they were limited to 30 or 25 frames per second. This could potentially cause flickering on the screen. Interlacing solved this. Each frame is split into an odd and even field. When played back on a television, the tv would project the odd field followed by the even field. When an image was moving, then one of the fields would be offset from the other. The image, however, would look smoother than if it had merely shown a single frame.
see also progressive scan