Fluo diving is when the coral/creature fluoresce in bright greens/yellows/reds/etc when a blue/UV light is shone on them.  Fluo diving makes night diving fun again...  You will see the reefs and creatures as you've never seen them before.  The blue light should be of 465nm to get the best effect, and you should have a yellow filter (fluo filter) over your mask/camera to get the most vivid colors.