Any system which has an equilibrium, or natural balance, will tend to vibrate around that equilibrium point. When the vibration is not too large, it is generally called "Simple Harmonic Motion" (SHM), a lovely name that makes one think about the regular vibrations in musical harmonies. Vibration is an essential ingredient in most everything - from the electrons in atoms, to the regular ticking of a pendulum, or the crystal in your digital watch... (In many cases, it's not really even possible to stop most such "harmonies" at some level)

In 1940, the newly built Tacoma Narrows Bridge in Washington State began to vibrate due to winds. The frequency of vibration matched its natural "modes", and the amplitude of oscillation got larger and larger. Within hours, the entire bridge had collapsed. It was a spectacular failure. It's not that the wind was so strong, it was the matching of natural vibration frequency that took it down.