It is assumed that you understand that all matter has a rate of vibration which is peculiar to itself. To have a rate of vibration is to have a rhythmical pattern of recurring periods wherein the energy of the vibration changes from one value to another. In such a situation, the frequency of a system is said to oscillate between certain maximum values. Two systems having the same natural frequency, when joined together in phase, resonance occurs with the result that the minimum and maximum values are reached simultaneously. As an example the fact that a vibrating tuning fork will set into sympathetic vibration, or resonance, another tuning fork of the same frequency.
Resonance involves an exchange between systems, a kind of mutual 'sensing'.
Every vibration has what is known as a fundamental frequency, plus its accompanying harmonics. Together with the fundamental frequency, there is an infinite series of harmonics, all vibrating at different Amplitudes, but all Reflections of the original Frequency. The frequncies of the Harmonics are all integral multiples of the fundamental frequency. The first Harmonic, following the fundamental, is double the fundamental, the next is treble the fundamental, and so on. We conceive then, of each living organism as radiating out into space, a frequency indentifying other aspects of its physiological and psychological existence.