In the aquarium nitrogen cycle, nitrosomonas bacteria feed on the ammonia from decaying food, fish waste, etc. The bi-product of the nitrosomonas bacteria is nitrItes.
Nitrobacter then feed on the nitrItes and the biproduct is nitrAtes.
Levels of ammonia and nitrItes are dangerous to most fish and only extreme levels of nitrAtes are very dangerous.
If you're cycling with hardy fish (most livebearers) then the ammonia and nitrItes will elevate and drop. When they constantly remain at 0, with a nitrAte level above 0, then it is completed the cycle. UNLESS, the tank is not fully stocked with fish, which will require you to add more fish, which will then continue with the cycle again.