The Busy Light or Channel In Use indicator is a feature of most radios which enables you to see if a shared channel is in use before you make a radio call.
Most licenced radio channels and all PMR446 radio channels (frequencies) are shared. Sharing means that another group of users within radio range also has the ability to transmit on the same frequency that you are using.
If you and another user transmits at the same time the chances are that neither of you will be intelligible to anyone receiving your calls. The call will sound garbled. So transmitting at the same time as others should be avoided.
You don't normally hear other users from other groups because your CTCSS settings keep your radio silent to their traffic.
But CTCSS prevents you hearing other user groups so a Busy light on your radio lights to indicate that the channel (frequency) is in use. If you see the busy light frequently illuminated then you should consider using a less active channel.