How old are you?

Something for the brightness old pc's had, those were really bright

You're mostly correct, they're monitor filters used to filter out the light reflected from the smooth glass surface of a CRT or flat screen display as well as the radiation emitted from CRT and LCD displays. Many also include a ground to dissipate static buildup. A secondary use for monitor filters is privacy as they decrease the viewing angle of a monitor, preventing it from being viewed from the side; in this case, they are also called privacy screens.