Here is cycling in a nut shell (sorry if you know some/all of it):
Fish create ammonia, too much ammonia can and will kill them... so when you cycle you get a little ammonia going.. and "In a new aquarium, this will rise rather quickly because the nifty little bacteria called nitrosomas aren't established yet. They break the ammonia down into nitrite, which is not quite so toxic as ammonia." Then nitrates form and "get rid of" the nitrites... then all is well.
Lets say for example you have a 10gal tank and you add 20 fish to it with out it being cycled. All of those fish are creating ammonia that isn't being destroyed by nitrites and nitrates... so they fish will then die from too much ammonia. In a cycled tank.. the bacteria is already there and doing its thing, so when you add a fish (and its ammonia) the bacterie takes care of it.
Make sense?