"CTCSS" stands for "Continuous Tone Coded Squelch System", a system that is used to avoid interference between different agencies within close proximity to each other using the same frequency. Each radio for a particular agency is programmed with a CTCSS code so that only those radios can hear and talk with each other and not a neighboring agency. 

The system involves an industry standard set of sub-audible tones for controlling radios and associated equipment. The sub-audible tone is added to the transmitted signal. The receiving radio is then set up to listen for this specific tone in the received and demodulated audio. If the matching tone is present, the squelch is opened up, allowing the audio to pass through to the speaker. If the tone is not present, then the radio remains silent, even though there is a signal on the frequency. This allows two or more agencies to use the same frequency (generally on a repeater), but not hear each other's conversations.