What is CTCSS?
CTCSS stands for Continuous Tone Coded Squelch System.
Interference-Elimination Codes, sometimes refer to Tone Squelch, TalkGroup or Privacy Code.
It works like Key and Lock on Door. Walkie Talkie transmits sub-audible tone (Key) and opens the door (Lock) on Receiver. For example: Group A on Ch1 no CTCSS, Group B on Ch1 Code1, Group C on Ch1 Code38. Group B won't be bothered by Group A (no Keys) C (wrong Key). Group A can hear all groups because no Lock (incoming door is open).
Keep in mind that this feature doesn't actually protect your privacy. As you see Group A who turned off CTCSS can hear all in the same channel. In addition, anyone who has chosen the same channel and code as you can still transmit to your radio.