Overscan is a behaviour in certain television sets, in which part of the input picture is shown outside of the visible bounds of the screen.
It exists because cathode-ray tube (CRT) television sets from the 1930s through to the early 2000s were highly variable in how the video image was positioned within the borders of the screen. It then became common practice to have video signals with black edges around the picture, which the television was meant to discard in this way.
Overscan usually occur when a PC is connected to an external display, such as an HDTV or TV. And the HDTV probably doesn’t show the whole picture on its screen. In fact, up to five percent of the picture can get cut off around the edges.
To fix the overscan problem, you need to disable overscan function. Most TVs have a way to disable overscan, just goto the menu of the TV's picture settings, if you see something called "Overscan", just turn it off.
If you don’t see that setting, however, it doesn’t necessarily mean it’s not available on your set—it probably means that the manufacturer decided to change the name to make it “easier to understand.” In that case, you’ll have to either keep digging and tweaking till you find it, or you can read the manual.
Here is a quick list of some of the most popular manufacturers and what they call overscan on their sets: