Any system has a resonance at some particular frequency. At that frequency, even a slight amount of energy can cause the system to vibrate.
A stretched piano string, when plucked, will vibrate for a while at a certain fundamental frequency. Plucked again, it will again vibrate at that same frequency. This is its natural or resonant frequency.
While this is the basis of musical instruments, it is undesirable in music-reproducing instruments like audio equipment.