A cavity resonator uses resonance to amplify a wave. The cavity has interior surfaces which reflect one type of wave. When a wave that is resonant with the cavity enters, it bounces back and forth within the cavity, with low loss (See standing wave). As more wave energy enters the cavity, it combines with and reinforces the standing wave, increasing its intensity.
In a laser, light is amplified in a cavity resonator which is usually composed of two or more mirrors.