Radio or TV signal interference
The answer is frequency or channel separation.

The electromagnetic spectrum, specifically the broadcast frequency ranges, are divided up and managed by different regulatory agencies depending on where you are in the world. The job of these agencies is to provide rules that govern what types of transmissions are allowed on different bands, and who is authorized to do so. Within each band, the rules also specify the separation between different transmitters.

The short version: Two stations within geographical proximity can operate if they are on different frequencies, sufficiently separated that they don't interfere with one another. This doesn't mean it never happens. Equipment getting out of calibration, lack of maintenance or tuning, and other factors can all cause unwanted interference.