Flips the polarity of the signal - every sample times minus one. This is sometimes called phase inversion or a 180-degree phase shift.
Polarity is the fixer for cancellation problems: when two copies of a signal meet (a mic pair, a parallel effect chain) and the sum goes thin and hollow, inverting one branch often restores the body. It is also a classic trick source: pan a mono sound to one speaker and an inverted copy to the other for a wide pseudo-stereo illusion, or build phase-based stereo encoding and decoding schemes.
No parameters - patch through it and the flip is the whole story.