LCD (liquid crystal diode) displays use electric charges to twist and untwist liquid crystals, which causes them to block light and, hence, emit blacks. The higher the voltage passing through the liquid crystals in a given pixel, the more fully those crystals untwist and effectively block light - all of which makes these pixels darker.
LCD displays are recommended for viewing in bright or well-lit environments. The one continual drawback for LCD is off-axis viewing, when black levels consistently drop.