If you've never heard of noise gate effects pedals before, you only need to look at the name to understand what they do. They're gates for the sound that goes through them, and just like a physical gate, most of these pedals allow you to open them to the exact width you need. In essence, a noise gate is the opposite of a compressor pedal: while the compressor sets an upper limit for a signal, the noise gate sets a lower limit. This allows you to filter out unwanted line noise to get a cleaner signal, which makes noise gate effects pedals popular in the studio (where they allow crystal-clear recordings) as well as on the stage (where they prevent static and noise from reaching your amp).
In many ways, you can think of a noise gate effects pedal as something like an after-the-fact humbucker on your pedalboard. They can help you achieve the sort of clarity you'd get from humbucking pickups even if you're using single-coils. That explains the name of the Electro-Harmonix XO Hum Debugger Hum Eliminator Guitar Effects Pedal - and that's just one of the top-selling pedals in this section, a roster that also includes the Boss NS-2 Noise Suppressor Pedal, MXR M-135 Smart Gate Pedal and Pigtronix Gatekeeper Noise Gate Pedal, to name just a few.
Some of the noise gates you'll find in this section are a little different from standard effects pedals - you could call them relatives. For instance, the Rocktron Buzz Kill AC Hum Eliminator Guitar Effects Pedal could go on your pedalboard, but it can also be used with lots of different pro audio hardware. For more examples, look at the 2-Channel Hum Eliminator and Hum X Hum Exterminator, both from Ebtech. Like the Buzz Kill, these are designed to clean up the power coming right from your wall outlet or AC adapter, which can add noticeable humming to your sound if left unconditioned.
Planning your biggest performance of the year, or heading to the studio to record your first album? Don't forget your noise gate effects pedals. They may not seem like much at first glance, but the improvements these effects units can bring to your tone might make the difference between a decent sound and an amazing one... and it goes without saying that "amazing" is the goal for any musical performance.