A vibrational mode is the standing wave pattern that give the highest amplitude vibrations with the least amount of energy input. If you vibrate an object at it’s natural frequency, you’ll get the highest amplitude during the vibration. Sometimes the amplitude (which is related to the energy of the vibration) that the object vibrates at is so high that the object will actually will tear itself apart. Here’s a video where the wind was blowing the bridge, which started a natural vibration in the bridge which tore itself apart.