Continuous Tone Controlled Squelch System (CTCSS) is used to keep your radios silent to other users sharing your frequency. (also calls QT/DQT, PL)
All radio channels are shared to some extent either sometimes locally (within radio range) as in PMR446 or nationally or internationally where geographic location helps prevents interference. Either way it is almost certain your radio would pick up other users now and then.
The main method to silence this interference is CTCSS. It works by setting all radios in your group to transmit a common sub-audible (67 to 250hz) tone whenever the transmit button is pressed. All radios in your group will only un-mute the speaker when this tone is received. The chances are other user groups will not be using the same CTCSS tone so your radio will be silent to their traffic.
All the radios we supply have CTCSS as standard except most Marine VHF and Air band equipment which does not normally use CTCSS for historical and safety reasons.