LCD backlights create the image that you see on the screen. They are a row of lamps or bulbs situated inside your monitor or TV behind the LCD.
When an image is created on your monitor, it is generated by the LCD segment of the screen device, and made visible by the brightly lit backlight.
Issues with LCD backlights aren’t uncommon, but usually affect older devices. If you have a new monitor with display issues, consulting your device documentation for warranty information should be your first step.