The NIR illumination is needed to get enough details from dark eyes. Infrared radiation is electromagnetic radiation with longer wavelengths than those of visible light, and is therefore invisible, although it is sometimes called infrared light. Infrared light falls just outside the visible spectrum, beyond the edge of what we can see as red.
In the images below, you can see that a light eye has more detail than a dark eye when photographed with a normal camera. But in the NIR illuminated version, all of the detail of the iris itself can be captured. So for iris matching, the image is captured with NIR illumination.
Once the image is captured, the iris is detected and converted to a binary number (ones and zeros). Imagine a spiral is drawn on top of the iris and the image unwound, so the iris image is now one very long line that is one pixel high. That image is converted into binary which makes it’s easy to compare and very accurate. This is a key advantage of the iris as a biometric modality - it's very fast and very accurate.