A noise gate is a type of processor commonly used in music production, a noise gate helps to reduce unwanted sound by only allowing the signal to be heard once it has exceeded a certain amplitude. Noise gates are also similar to compressors in that they reduce the volume of unwanted audio but they differ in the fact that compressors will take affect on the signal once it passes over the given threshold whilst noise gates will reduce signals that are under the set threshold. Once you set the volume threshold of the noise gate the gate will then open when an audio signal passes this threshold, allowing the audio to come through the gate and be heard, and the gate will close again once the signal goes below the threshold. How much a noise gate reduces the volume of a signal depends on the ratio which you set; the higher the ratio and the more the gate will apply volume reduction to the signal once it goes below the threshold. You can also modify the rate at which the gate will open by controlling the attack and release, with some noise gates also having a hold control. These different controls all have different meanings, attack controls the rate at which the gate opens, release controls the rate at which it closes and hold controls the time that it takes for the gate to begin to close after the signal goes below the threshold. Noise gates are used in the mixing process to remove unwanted background noise, for trimming off transients, and also just sculpting the sound to how you want it.