Fluorescence is seen under Ultraviolet light in colors such as blue, white, and in some cases yellow, red, orange, and green, with an intensity of very slight, faint, medium, strong, and very strong.
To understand better what is fluorescence (without touching the chemical part) imagine taking a cup of water and pouring some milk in to it, that milky color is what fluorescence looks like.
UV light is everywhere, the sun produces UV radiation which most of it gets blocked by the Ozone Layer, but UV light is presence everywhere.
Fluorescence has nothing to do with clarity or color, it is possible to see a D color, FL clarity with very strong fluorescence.
Does fluorescence affect the value?
Depending on the appraiser that you ask, some will say yes and others no.
Investment Diamonds shouldn't have fluorescence, for the simple reason that some appraiser say it has an effect, always remember Investment Diamonds have to be bulletproof to all bullets, not just to some, sometimes the words of one appraiser can destroy your investment, so why give him the opportunity.