resonance

Every object has a natural frequency. If a fluctuating force is applied to the object with the same frequency as the object's natural frequency, the object will vibrate very strongly. This is resonance. If the frequency of the force is different, the object's vibration will not be so strong.

An example is soldiers marching in step over a footbridge. If the frequency of their stepping motion matches the natural frequency of the bridge, it could vibrate so much that it would break.