Radio signals are broadcast at a particular frequency which is called a carrier wave. The carrier wave is usually a simple sine wave. In order to send information such as music or voices, the carrier wave has to be modified. The information is in the modifications.
There are basically two ways that a simple sign wave can be modified in order to encode the information. One of those two ways is to modify the amplitude of the sine wave. The fluctuations of the sine wave amplitude can be decoded by the receiver and turned back into the original information.
The second method of encoding information is to modify the frequency of the carrier wave. Again, fluctuations of the frequency of the carrier wave about its primary frequency can be decoded by areceiver in order to recver the information which is encoded.