Cycling is the act of establishing the beneficial bacteria colonies before (or while) stocking a tank, so the filter can neutralise ammonia and nitrite as fast as the fish produce them.
A fishless cycle (adding an ammonia source to an empty tank and waiting for the bacteria to catch up) is the kindest method — nothing suffers while the bacteria grow. A fish-in cycle uses hardy fish already in the tank and demands frequent water changes to keep ammonia and nitrite near zero.
A tank is cycled when it can process a full dose of ammonia down to zero ammonia and zero nitrite within about 24 hours. Test, don't guess — the water tells you, not the calendar.