What is overscan

[dolfnman]

I'm a fairly new owner of a Tosh 50HX81. I am learning slowly about tweaking the set. I have learned to do the 56 pt. converengence and while utilizing the VE DVD I saw the set was not centered, so I corrected that. But what is overscan? I hear people talk about reducing overscan. I see on the VE disc a overscan image with the white lines, but am not sure what I should be doing with that. If I adjust the HIT and WID I see the numbers on the image get a little fuzzy, is this what I am supposed to be looking at to judge overscan. Any tips would be appreciated

PS
I always right down all my settings before test different parameter.