Resonance, as an acoustical phenomenon, describes the ability of a physical enclosure to amplify incident sounds at particular resonant frequencies. The vocal tract has time-varying resonances which act to draw extra power from the glottis (or other energy source) at those frequencies; a portion of this extra power is projected from the noise and mouth. The radiated vocal sound then may have formants corresponding to the resonant frequencies.
In electronic music practice, resonators, usually called resonant filters, also amplify sounds selectively to relatively enhance one or more resonant frequencies; in this case, though, an external voltage source can provide power needed for amplification. A typical resonant filter has one resonant frequency, whereas physically resonant bodies can have many.