CF in AC stands for "clean filter". When the air filter in an air conditioner becomes too dirty, it can cause the AC to display the CF error code as a reminder to clean the filter. The air filter functions as a channel that allows air to enter the system, and when it is too dusty, enough air cannot enter the system, which can cause an issue with the temperature or noise. It is recommended to clean the filter every two weeks or as soon as the CF display code appears on the indoor unit display.