The phaser is used to create a classic sweeping sound suitable for pads and guitars, but can be used for more extreme effects too.
The phaser effect units work by creating a complex frequency response containing many regularly-spaced "notches" in an incoming signal and combining it with a copy of itself out of phase. The positions of the notches are modulated over time by the use of a low-frequency oscillator.
The phasing effect is a kind of hollow "whooshing" sound reminiscent of a flying jet airplane.
A stereo phaser uses two identical signals modulated for the left and right channels with one usually being a quarter-wave out of phase.
The phaser is best used as an insert effect.
Clap without phaser effect
Clap with phaser effect