Duke!......you said poop!
He-he.....he said poop.
Anyway, Ammonia is a chemical, it results usually from a breakdown of other chemicals. It happens in us humans too, but thanks to our livers and kidneys, we dispose of it rather quickly.
Ammonia in a fish tank is usually the chemical breakdown of fish food, decaying material, and of course, fish poop. Ammonia, at high enough levels, is very deadly to fish, and also humans for that matter. Fortunately for humans, we don't live in a tiny closed ecosystem. Which is what a tank is. Fortunately for us tank lovers, certain bacteria take over and eat the ammonia, but it is a very delicate balance, hence the nitrogen cycle.
HTH :happyfish