With most non-OLED displays available today, there are two main components responsible for generating the picture you stare at: the LCD panel and the LED backlights.
The LCD panel (short for liquid crystal display panel) is what contains your standard RGB pixels that will form a picture when light is passed through them in a specific manner. The LED backlighting is what illuminates this picture and makes it visible to you. Some TVs will have a ring of LEDs around the edge of the TV that light up the picture, while higher-end TVs will have a full grid array that can make specific “zones” of your TV glow while leaving others dark.
The best OLED TVs takes this latter idea further as there is no difference between the LED and the pixel. They are one and the same. This means that you can illuminate each individual pixel and have an insane amount of control over which parts of the screen are full of brilliant color, and which parts are full of stark black nothingness.