Cycling is the process of establishing colonies of beneficial bacteria in your filter. Fish waste is basically ammonia and is toxic to your fish. Ammonia is kept from killing our fish by developing colonies of these bacteria that convert ammonia to nitrite. Nitrite affects the blood's ability to carry oxygen; so, it can also be toxic. Luckily, a second set of bacteria will colonize and convert nitrite to nitrate. Nitrates aren't generally considered toxic until they reach higher levels. Doing regular, partial water changes, using plants, or both removes nitrates. It usually takes 4-6 weeks to get large enough bacteria colonies to keep up with the ammonia and then nitrite levels. This is why it's always recommended that you start with just a few hardy fish. Feed them small amounts (what they eat in 30 seconds) once a day or even once every other day until your ammonia and nitrite levels have risen, and returned to 0. Then you can slowly start adding fish every few weeks until you get to the expected bio-load for the tank. The term bio-load refers to the maximum amount of ammonia your bacterial colonies will need to process when your tank is fully stocked. In other words, the maximum amount of fish, food, and waste your tank can handle.