fishless cycling is when you add ammonia to the tank to create the good bacteria you need for your fish to thrive in the tank, when you do a fish cycle the fish waste is producing the ammonia needed to create a colonie of good bacteria, but you will need good hardy fish like danios or platies for the fish cycling and you will need to add fish slowly after the tank is fully cycled. This is why fishless cycleing is much better because you would be able to add alot more fish at one time because there will already be a good colonie of good bacteria built up in your filter.
If i anything i said is wrong then i apologize and someone correct me if there is anything wrong