The 1st thing to understand is the water doesn't contain the bacteria in the "cycle".
The bacteria grows,colonizes the hard structures.Gravel/decor/filter media/sides of glass.
You can feel the bacteria.It is the slimy coating that is on everything in the aquarium and filter.Now rub your hands together,the water rinses the slimy feeling away. So,to keep the water from rinsing the bacteria away,it develops a gel like glue substance to keep the colony stable.That's the slimy substance you feel.
2nd thing- All living things produce waste.Wastes decay into
ammonia.A bacteria forms to break the ammonia down to nitrites.Another type of bacteria develops to break nitrites down to nitrates.This development of the bacteria is the 'cycle'.Once the bacteria colony is large enough to reduce the ammonia and nitrites to 00 in a given time frame,is the end of the cycle.
first the wastes break down and you start reading ammonia in your tests.
next the ammonia is broken down and you start reading nitrites in your tests while the ammonia levels drop.Then you start reading nitrates while ammonia and nitrites drop. ammoni and nitrites will eventually be 00 and nitrates will increase.Your cycle is complete.
Meaning the bacteria colony is large enough to break everything down to non-harmful components.(almost)
3rd- Nitrates are not harmful,up to a certain amount.(20ppm) after that,they start becoming toxic in increased concentrated amounts.Changing the water,removes the nitrates so they are not harmful.Plants can utilise nitrates as food.The amount of nitrates in the water determines how much water needs to be changed,and how often. As the
nitrate levels increase,so should the amount and frequency of the water changes.