Resonance is the exaggerated oscillation of a body if the frequency of the driving force to the body is equal to the natural frequency of the body. Below and above this particular frequency nothing out of the ordinary occurs but at the exact frequency the amplitude of the oscillations can dramatically increase.

Armies that walk across bridges do not walk in their usual routine way as each foot step made creates a driving force for the bridge. If the frequency of the armies' march equals the natural frequency, then the bridge can begin to resonate and collapse due to the structure being deteriorated.