Frequency modulation in its simplest form involves The repetition rate of some periodic vibration pattern, usually described in terms of repetitions per second. 

If, for example, we say that a musical tone has a frequency of 300 hertz, this indicates that the waveform repeats itself 300 times each second. Frequency is inversely related period as given in the following equation: 

Frequency = 1 / period
In the example above the frequency of the signal can be determined from the two measurements given: 
3 cycles / .1 second = 30 Hz
The discrete frequencies that make up a complex timbre can be found using Fourier analysis.  Frequency is correlated with the subjective sensation of pitch
See Also:
frequency discrimination    human range of hearing