the heat range on a plug refers to how hot the plug accually gets..
a hotter plug will burn off build up on the spark plug to prevent fouling..
but if you get a plug thats too hot and you ride on a hot day that hot plug will be a hotspot for preignition.
the use of a colder plug is for when you start getting into some higher performance like with higher compression and so on..
because the engine is accually running hotter you want a colder plug to help pull heat away from the engine..
basicly you want to run the coldest plug you can get away with, without fouling out the plug..