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.