The final product of the Nitrogen Cycle is nitrate. This is a compound that is used by plants as food, and the plants convert it into oxygen. Even if you have a heavily planted aquarium, nitrates will build up faster than most plants can use it. This will require regular water changes.
One important and often overlooked aspect of setting up an aquarium is tank cycling. This is the process of getting the good bacteria to grow in your aquarium. The Nitrogen Cycle begins when ammonia starts to build up in the tank and bacterias that convert ammonia into nitrite starts to develop. Once the nitrite starts to build up, bacteria that converts nitrite into nitrate starts to grow. Nitrate is reduced through regular water changes and plants utilizing the nitrate for food.