For questions about the resonance phenomenon in music.
A system is said to be "resonant" at a given frequency if, submitted periodically at this frequency, it produces a response which increasing with time, usually in an exponential manner if not dampened.
Resonances are everywhere in music instruments and acoustics in general: for strings instruments the sound produced will be a the resonance frequency of the string, for wind instruments it will be the resonance frequency of the air column in the instrument…
However, sometimes resonances are not desired. They are for instance responsible for the feedback effect.