Millions of TV viewers every day are blissfully unaware that they're missing the big picture. In the early days, a little cathode-ray gun inside the TV, worked by painting one line of the TV picture, then turned around and heading back the opposite direction. To make sure the screen was painted and edge-to-edge, these early TVs were programmed overshoot the edges of the screen or, to use the technical term "overscan" the screen.
Television technology is much better now but even modern television's exhibit overscanning, the amount is that you may be missing as much as 10 percent of the picture beyond the left and right edges, and often the top and bottom too.
To avoid text-chopping problems in iMovie, to turn off the QT Margins check box. The TV safe area is not an issue if you plan to convert your iMovie work into a QuickTime movie, because it has no such complications. That is why the check box is worded QT margins meaning assume that the movie is going to be shown in QuickTime. When the QT Margins check box is turned on the text-size slider lets you crank up your fonts size a few notches higher. Summarized from David Pogue's "iMovie 2 - The Missing Manual"
What Can I Do About It?
Well not much, unless, your willing to deal with seeing a black box in to video. If so then you can use this Quicktime Pro mask trick.
How do I know if my video is going to have an overscan problem?
I know of one free plugin from Stupendous that will tell you, but there may be others.