Fish, decomposing fish food, and decaying plants produce waste that contains a lot of nitrogen in the form of ammonia (NH3), which bacteria then break down into chemicals called nitrites.
Cycling is just building up good bacteria in your tank via the nitrogen cycle. Fish, decomposing fish food, and decaying plants produce waste that contains a lot of nitrogen in the form of ammonia (NH3), which bacteria then break down into chemicals called nitrites. Ammonia and nitrites are toxic to fish, so you want to build up the bacteria that breaks those down further into nitrates, which are only harmful in large amounts that can be prevented with water changes. Note the difference between nitrItes and nitrAtes.