Liquid Crystal Display or LCD refers to a type of screen technology that lends towards thin screens. This has gaining popularity in the television industry, especially in HDTV, creating thin, lightweight television sets. You'll find more info on LCD technology at Wikipedia.
Using the technology to create large displays like computer monitors and TVs is a relatively recent turn of events. LCD technology is probably most familiar to use as digital watch faces, calculators, car instrument panels, radio displays, personal CD players.
In fact, just about anywhere that needed a versatile user display. The bare LCD panels are available to purchase and Myth incorporates a program, mythlcdserver which allows the display of useful information while Myth is running. For instance it will display the current time, progress through a recording or while in MythMusic it can display information about the currently playing track.