See the illustration below. As long as the input signal (red line) is below the threshold (gray line), the output signal is zero (green line). Simply put, a noise gate is either open (sound passes through) or closed (sound is not allowed to pass through). A noise gate allows sounds above a volume threshold to pass through while it blocks sounds below that threshold. Instead we're hiding the noise during quiet parts of a track.
With a noise gate we are not actually removing noise from sounds. In this Fruity Love Philter tutorial, we'll demonstrate how to remove this unwanted noise. Perhaps the noise of individual sounds is hardly audible, but if you have multiple tracks the noise gets magnified, which can make your mix sound muddy. If you want a clean, crisp sounding mix it is important to reduce the background noise from your sounds and samples.