A noise gate VST is an audio effects plugin that doesn’t allow sound through the gate above or below a set threshold. Think of high transients.  The level at which the noise gate kicks in is controlled by Threshold. If the noise gate is open and the sound is above the threshold, it will let the sound go through.  However, the noise gate closes if the sound is below a set threshold so that no sound goes through. Likewise, there are attack, release and hold controls.  While Attack controls the speed at which the noise gate sets in, Release controls the length of time the noise gate will fade out for. And, hold controls the duration till which the noise gate remains closed.