Quantum dots are semiconductor particles small enough that their size affects the color of light they produce. In display designs, they can be used to convert light from another source into a narrow band of red or green.
Tiny particles, tunable light
When light excites a dot, it can re-emit at a characteristic wavelength. By selecting particle materials and sizes, engineers tune the output and improve color purity. A display still needs a way to supply energy and control individual picture elements; the dots alone do not create an entire image.
What the screen still needs
Heat and the surrounding materials influence brightness and stability. Not every display advertised with quantum dots uses the same architecture: some use dots as a conversion layer with an LCD backlight, while other approaches integrate them differently. The useful idea is precise color conversion, not a promise that every screen with the label performs identically.
Related: how an LED turns electricity into light.