The Pigtronix Gate Keeper is a noise gate designed to keep all unwanted noise from your rig: it basically shuts down the signal when you stop playing and opens up again as soon as you hit a note.
Just two knobs control the action - the threshold knob controls how much signal is needed to open the gate, while the release knob determines how long it takes the gate to close after it's fallen below the level set by the threshold, albeit with a limited range.
A mute LED shows when the signal has fallen below either threshold. The pedal certainly works fast and efficiently and doesn't seem detrimental to tone, but you have to be careful to get your knob positions just right, especially so if you're playing sustained notes and don't want them truncated.
For certain types of music, perhaps high-gain metal riffing and chords, it can certainly keep your sound tight and controlled.
If you have a hissy or hummy rig, this can unobtrusively keep the noise at bay.