Noise gates take an audio signal from one source, instrument or vocals, and regulate that signal to a second source, usually a speaker. Signals under a set volume are "gated" so they do not pass to the second source, while signals over a set volume do pass through the gate.
Noise gates can either be a software program or an actual piece of equipment that the signal is then sent through.
Noise gates are used in recording studios as well as in live stage shows for both recording, mixing and adding effects.
Noise gates can also be used to reduce noise that appears on recordings such as amplifier hiss or unwanted microphone sounds.
Phil Collins used a noise gate to get the haunting vocal sound on his hit single "In the Air Tonight."