If you don't know what it is, it's a low pass filter that cuts the subharmonic frequencies. Sounds simple, but it cuts those "thumps" out without affecting the rest of your tone. That's where it's magic lies. So you get a signal that feeds more cleanly into octave pedals or other effects -- less glitching. And it makes your amp work less because it doesn't have to try to reproduce the thumps you can't hear anyways. Sound guys love you, your amp and speakers love you.
For a visual demonstration: https://www.youtube.com/watch?v=TacaSl6dimU
(Hope you don't mind me adding to the post, I like mine.)