I can try, par stands for photosynthetically available radiation. It is basically tells us how much of the light is actually available to be used by the photsynthetic algae in a coral. Usually people will say low light corals can grow with 50-100 par, medium light 100-300, and high light corals needing above 300 to grow. However, most ppl don't have ready access to a par meter so it is hard to determine just how much par a fixture puts out at different depths in the aquarium.
20 Gallon SPS reef
29 Gallon FW "fat catfish" tank
20 Gallon Long Dart Frogs "Zig" and "Zag"