There are probably a million bad Star Trek jokes that could come from the word Phaser, but a phaser pedal is a unique and expressive effect for just about every instrument. A phaser pedal creates a swirling sound by splitting an audio signal into two, one of which is unaffected, the other passes through a filter that preserves its amplitude and shifts its' phase through what's known as an "All Pass Filter". This filter is typically controlled via oscillation, A.K.A LFO. The two signals are then mixed back together where they cancel each other out.
The phaser has become the signature sound for several aritsts and songs throughout the years. Van Halen, Live, Radiohead, Genesis, Steely Dan, Billy Joel, the list goes on and on.