Law enforcement officers depend on clear and constant communication with each other to capture criminals and assist each other, and much of that communication takes place over police radio frequencies. Federal officials have reserved a number of channels on broadcast frequencies specifically for the use of police, fire and emergency personnel, and most civilian radio sets do not have the required technology to broadcast on them.
Most police radio frequencies are well known, and owners of police scanners can tune into them and listen to the talk between officers and the calls that are dispatched to them from 911 communication centers. Because this chatter can give a clear picture of where officers are located and when and how they are responding to calls, it is usually illegal to carry a police scanner in a motor vehicle, because criminals use this information to gauge police response and avoid arrest when committing crimes.
Not all police radio frequencies are open to public listening. Some are reserved for sensitive operations, and can be digitally scrambled to prevent eavesdropping by the general public. A dramatic example might be radio communications between Secret Service agents protecting a VIP. Obviously any threat to the dignitarys life would be compounded if a potential assassin knew their route, plan, and real-time location at all times, so this kind of communication would be scrambled and nearly impossible to monitor.
Many people find it very interesting to listen to the radio traffic on police radio frequencies. If one listens carefully and understands the jargon, it can produce a fairly accurate picture of the average police officers day, as well as inform the listener on the strange and dangerous things which they face regularly while the public has no clue as to what is happening in their own communities. Sometimes the radio calls can be quite humorous. Sometimes they can be terribly tense and chilling. Police officers are conditioned to respond to radio calls at any time they are on duty, so the sound of a radio broadcast will generally bring them to attention immediately.