All systems have natural frequencies at which they prefer to vibrate. An external energy of the proper frequency and with the proper timing or phase near a system’s natural frequency will cause the system to vibrate with or resonate with the external energy. A simple example is the common child's swing. To make a swing move in a larger arc, you must push it at just the right time (phase) and this corresponds to the swing’s natural frequency. Another example is two guitars. If a string on one guitar is plucked, the same string on the other guitar will vibrate, even if the guitars are many feet apart. Systems driven at resonance can absorb large amounts of energy. A soprano holding the resonant frequency of a crystal glass can transfer enough vibrational energy to the glass that it shatters.
I was once swimming with Liberty Dolphin in Florida. In the murky water I accidentally jammed my elbow into his blowhole area. I think it hurt him. He came around in front of me, and at about 2 1/2 feet from my chest, began to put out a high intensity, fairly low frequency sound, probably less than a thousand Hz. First, at low power, I could feel the water vibrate. Then his power increased and I felt the water in front of my chest get warmer. Then, Liberty increased power again and I felt my chest wall get warm. Then he changed frequency and soon, all my chest hairs were vibrating- even in the water, so strongly that my whole chest tickled with an intense buzz. Liberty stopped his sounds at this point. I had the distinct impression that he was just showing me part of what he could do, something like: "This is just a demonstration, I could do a lot more, I'm just warming up"