Organic Light Emitting Diode is a display technology that uses organic molecules (diodes) to emit light. These organic diodes are placed between two transparent electrodes, allowing each diode to light up or switch off individually.
In comparison, a Liquide Crystal Display (LCD) uses liquid crystals and a single backlight. So, unlike an OLED, an individual pixel cannot be switched off completely. As a result, this pixel still emits some amount of light that compromises the image quality.