Fluorescence
Light as we know it is a very small part of what scientists call the electromagnetic spectrum of waves. There are many different types of these waves from radio to the dangerous gamma rays. The only thing special about light is that our eyes can detect it.
Close to light but with more energy and invisible to our eyes is ultra violet (UV) radiation. The most obvious effect of UV is sun tan (or burn if you are careless). However, there are some materials that will absorb the energy from UV and re-radiate it as light. This gives an extra visible boost of light which we call fluorescence. This is why fluorescent materials such as wool or chenille glow in daylight.
For a fly fisher one should remember that although water is transparent to light, UV can only penetrate a few inches even in clear waters so it is unlikely that your fluorescent fly will have any difference to a normal fly when fished other than close to the surface.
If you have one of the small UV forgery detection lamps you can observe these effects with a jar of water.