Radio by its nature is not secure. A radio signal can be picked up miles away and eavesdroppers can listen in.

If you dont want others to listen to your radio conversations you have a number of ways preventing them, each with increased levels of security.

Defeating Casual eavesdroppers.
Use Guarded speech and Code words. Not saying anything meaningful to the eavesdropper is the simplest solution although relies of good staff practice and agreeing a common list of codewords (which may need to be changed occasionally).

Use Scramble. (analogue radios only). Scramble is a simple and effective way of defeating the casual listener especially if it is only used on per-secure-call basis rather than for every call. E.G. the call is made 'in the clear' then the sensitive information passed in scramble returning to clear comms afterwards

Use digital radios. These are much more difficult to eavesdrop with common scanners although in time common scanners will be able to decode common digital systems.

Defeating eavesdroppers of intent.
Most higher-end digital radios now come with an encryption facility. Each radio is programmed with a long 'Key' and the transmission is encrypted with this key using. There are various levels of this full encryption technology each progressively more secure and more expensive. A typically example is the AES encryption algorithm.