cycling is actually a reference to the Nitrogen Cycle. It is the build up of the bacteria population to consume the waste into less poisonous products. Without cycling the tank, the fish are basically swimming around in their own waste. Which is ammonia. Given time, the bacteria population becoumes sufficient to convert this ammonia into nitrites and then into nitrates. That is the nitrogen cycle. In a new tank, the bacteria cant keep up with the waste produced by a fish so ammonia bulds up and becomes toxic to fish. That is why we cycle our tanks with some fish food or a raw table shrimp. When the food rots, it provides a food source for the bacteria so that when we do add fish, they arent put in any danger from a build up of waste.