Here we have six pendulums that form three pairs of equal lengths. All are suspended from the same supporting rod, which is free to move back and forth. Now, set one of the pendulums in to motion. You will notice that another pendulum of the same length starts oscillating with maximum displacement.
This is because of the maximum transfer of energy of motion of one pendulum to the other of same length. This is the principle of resonance.
In this model three pairs of pendulums each of different lengths are suspended from a supporting rod. If one of the pendulum is set into oscillation, its pair, whose length is same as the former also starts to oscillate. This is all because of the supporting rod which itself starts to move back and forth when the pendulum is set into oscillation. This motion of the supporting rod transfers the energy of motion of one pendulum to another pendulum of the same length to a maximum extent. Due to this transfer, the other pendulum is set into oscillation, gradually covers greater distances from its rest position and reaches maximum when there is a maximum transfer of energy. By that time, the first pendulum would have almost come to rest. Now the transfer takes place from the second pendulum to the first one and this goes on till the second pendulum comes to rest. Thus, the cycle continues.
The scientific phenomenon behind the transfer of energy is known as "Resonance".
If the pendulum is pulled to one extreme and released without pushing, it makes a certain number of oscillations in a unit time. This number is a constant for a given pendulum and is known as its natural frequency or resonant frequency. This resonant frequency depends on the length of the pendulum. Longer the pendulum, lower the natural frequency.
If there is a connecting medium between two objects of same length, the motion of one object brings about motion in the other. This is resonance. The transfer of energy is then complete and maximum. The transfer takes place even between the objects of different natural frequencies. But, it will not be maximum.