Tanks need good bacteria to eat up the ammonia that the fish give off in their droppings. Good bacteria grows naturally over time with 25% water changes to the tank. The water changes insure that there isn’t too much ammonia. Adding a new fish to an uncycled tank means more ammonia. High levels of ammonia will kill fish rapidly. It’s very complicated, but that’s the basic idea of cycling. Like if someone gets a brand new tank and new fish as a gift. There are new fish and no good bacteria. A lot of the time their fish end up dying.

Sorry if I'm stating something you know already. I know a lot of people do, but it’s important to know.