How does resonance store vibrational energy? In the wiki article, it is written that in resonance, maximum amplitude is possible as vibrational energy is stored. 
What does that statement mean? How is energy stored so that max. amplitude occurs? Isn't energy stored in other forced vibration?
 A: When a mass-spring system moves, kinetic energy is 'stored' as momentum in the moving mass which is then converted to potential energy in the compression of a spring and back to kinetic energy as the spring extends, moving the mass back again. This periodic conversion of energy from kinetic to potential and back again is known as 'resonance' and occurs at a characteristic (natural) frequency, dependent on the mass and stiffness of the spring.
When the system is subject to periodic force (vibration) it will 'respond' by oscillating. If the frequency of vibration happens to be the resonant frequency of the system, it will 'reinforce' the natural frequency at which potential energy is transferred to kinetic energy within the system. That is, even if the applied force (vibration) is small, if it is applied periodically at the time when the energy already stored in the system is being converted to kinetic energy, it will 'add' to the amplitude of the response.
It's like pushing a child on a swing: if a small push is applied periodically at the right time, each push adds to the already (natural) motion of the swing so that it moves with maximum amplitude.
