What the term describes
Fluorescence is visible light emitted by some diamonds when exposed to ultraviolet radiation. It is not the same as the flashes of light produced by the cut, and it is not a substitute for a colour or clarity grade.
Avoid a blanket rule
The practical importance varies by stone and viewing conditions. Read the recorded intensity and inspect the actual diamond in appropriate lighting. Do not reject or prefer every stone solely on this one report entry.
Sources and further reading
This English edition draws on the source material referenced below.