Questions about Frequency Modulation, the analog modulation scheme commonly used for voice on VHF and UHF frequencies.
In a radio transmitter, a sine wave at a particular frequency, the carrier signal, is modified by incorporating a second signal, the modulating signal, that contains information to be transmitted. This process is known as modulation. In the radio receiver, the process is reversed to recover the modulating signal; this process is called demodulation.
For analog signals such as voice, frequency modulation (FM) is a modulation method in which the frequency of the carrier signal is varied in accordance with the instantaneous amplitude of the modulating signal. FM was invented by Edwin H. Armstrong in 1935, and has been popular in amateur use on the VHF and UHF bands since the 1950s.
More information about FM is available on Wikipedia.