The Resonance Effect is basically “the tendency of an acoustic system to absorb more energy when the frequency of its oscillations matches the system’s natural frequency of vibration (its resonant frequency) than it does at other frequencies.” Videos after the jump.
A resonant object will probably have more than one resonant frequency, especially at harmonics of the strongest resonance. It will easily vibrate at those frequencies, and vibrate less strongly at other frequencies. It will “pick out” its resonant frequency from a complex excitation, such as an impulse or a wideband noise excitation. In effect, it is filtering out all frequencies other than its resonance