Used to refer to equipment (usually speakers) to describe putting a thin layer of metal (typically lead) on the inside. Why? Because speakers have big magnets inside them. And cathode ray tubes (such as found in TVs and computer monitors) work by firing electrons at the screen. The magnets from the speaker deflect the electrons, with the net result that you get funky colour effects on the screen. Cool for about five minutes, until you see your favorite pop starlet with a green face.

So, when putting speakers next to a monitor or TV you should get magnetically shielded ones. This prevents the magnetism from deflecting the electrons, so you get a nice clean image.

WARNING: If you bring magnets close to the TV screen, they can permanently damage it. This will happen if you magnetize what is called the "aperture grille" on the inside of the monitor. It's very hard to demagnetize, but you can do it if your CRT has a de-gauss button.