To explain it in other terms: A quantum dot is an artificial molecule. Standard semiconductors of the type used to create light behave differently when they are very, very tiny. That is because they then fall under the rules of quantum mechanics instead of classical physics. Large masses of stuff behave in a different way than individual bits of it.

In the case of quantum dots, by creating individual light-emitting molecules, the emission can be held at a single wavelength of light, and therefore all of the energy that you put into it comes out in the way you want it, making a bunch of quantum dots more efficient than a similar amount of bulk material. Also, each dot can be used individually as a light sources for optical transistors on a chip to create optical computers and cool stuff like that.

One of the ways to make quantum dots involves creating a layer of semiconductor on a substrate, like for a regular microchip. The way the process is different is that they use materials with different shrink rates, so that the top layer shrinks faster, causing the semiconductor to crack up and coalesce into individual molecule-sized bits.