CTCSS is a system that has been invented to try and separate out different types of radio traffic on a single channel. The best way to think of it is like a filter that only will allow you to hear transmissions that are meant for your group of radios.

By tuning on the CTCSS filter, when a transmission is made on your channel not intended for your radio group, your radio will not allow the user to hear that broadcast.

For example a group of workers at a petrol station and pizza delivery are on the same channel.

The petrol station users and pizza user have different CTCSS filters set.

This will mean that only the pizza company can hear “pizza” related talk and not any of the conversations from the petrol station users. Conversely the petrol station workers will only hear broadcasts from other petrol station workers.

If someone is on your FHZ with no CTCSS or filter set, they will be able both the petrol station and the pizza delivery broadcasts. CTCSS is a completely insecure protocol.