The nitrogen cycle never stops. As food and fish waste break down it causes a spike in Ammonia in the fish tank. The beneficial bacteria that live in the LR and sand bed (not in the water column) convert the ammonia into nitrite. The nitrite then gets converted to nitrate. High levels of ammonia can be very harmful to your livestock. Nitrite can also be toxic. Nitrate, too, can be harmful, but to a lesser degree than ammonia and nitrite.
Two of the most common ways of ridding the water column of nitrate is through regular water changes and also through harvesting macro algae (commonly grown in refugiums.)
Corals like nitrate in the water. Soft corals, like Xenia for example, can handle higher concentrations of nitrate than, say, SPS corals. While SPS corals can tolerate levels of nitrate, often times the levels that they are healthiest with are not within the limits of our tests.