what is FM (Frequency Modulation)?

Answer 1

The signal (audio/TV,...) is carried by changing the frequency of the carrier signal. This contrasts with Amplitude Modulation (AM) that changes the amplitude of the carrier signal. AM is used in longwave, medium wave and shortwave transmissions (< about 50Mhz carrier) FM is used above this. Decoding FM is more difficult but FM is much more immune to radio interference and noise (you've only got to compare a MW radio station with an FM one)
FM radio stations started in the 1970s. I think TV has always been FM

Answer 2

Frequency modulation (FM) is a method of impressing data onto an alternating-current (AC) wave by varying the instantaneous frequency of the wave. This scheme can be used with analog or digital data.

In analog FM, the frequency of the AC signal wave, also called the carrier, varies in a continuous manner. Thus, there are infinitely many possible carrier frequencies. In narrowband FM, commonly used in two-way wireless communications, the instantaneous carrier frequency varies by up to 5 kilohertz (kHz, where 1 kHz = 1000 hertz or alternating cycles per second) above and below the frequency of the carrier with no modulation. In wideband FM, used in wireless broadcasting, the instantaneous frequency varies by up to several megahertz (MHz, where 1 MHz = 1,000,000 Hz). When the instantaneous input wave has positive polarity, the carrier frequency shifts in one direction; when the instantaneous input wave has negative polarity, the carrier frequency shifts in the opposite direcetion. At every instant in time, the extent of carrier-frequency shift (the deviation) is directly proportional to the extent to which the signal amplitude is positive or negative.