Acoustic resonance is a physical phenomenon in which an object begins to vibrate with more energy after being exposed to its natural frequency or a multiple of its natural frequency. In the case of acoustic resonance, this produces amplified sound.
A natural frequency is the frequency at which an object tends to vibrate when disturbed. It is due to the physical properties of the object. Resonant frequencies exist for all sorts of objects, but the concept is especially important for music production and acoustic instruments.
When an object with a particular natural frequency is exposed to this frequency by an outside force, it resonates. Resonating causes the frequency of the vibration to amplify, or increase in energy. In the case of acoustic instruments, this amplification makes the sound produced by the vibration louder. This phenomenon allows for the sound from the instrument to become loud enough to be heard by an audience, even without electronic amplifiers.