A noise gate is an audio processing tool generally found in pro audio situations which is used as a method of noise reduction. The noise gate is generally implemented with a user configurable threshold that determines when the noise gate will "turn on". When the audio input signal falls below the defined threshold, the noise gate closes, in effect preventing any sound from getting through. When the audio signal exceeds the threshold, the noise gate opens again, letting audio through.
The primary concept behind a noise gate is that when an audio signal is present it masks any noise, but when no audio signal is present, noise may be audible so the audio "pipeline" is shut off which prevents any signal (noise included) from passing through the device. A noise gate can be implemented as a stand alone device, or in conjunction with an audio compressor, limiter or other dynamic effect.