Overscanning is when the video signal is slightly larger than the viewable area of your screen. As an example, underscan would leave a small black border around your picture where the video wasn't quite big enough to fill your screen. Think of it as a very small picture zoom. CRT TVs almost always have overscan because the actual picture tube extends inside the plastic cabinet. I have a 2.5inch portable RCA TV that has no overscan. I can sometimes see the SMTPE timing marks from recorded video on the top of the screen during local news and various shows.