N-acetyl Cysteine, also known as NAC or N-ac, is an acetylated form of L-cysteine, an essential amino acid. It has antioxidant, and anti-inflammatory properties and is a precursor of Glutathione, the body's most powerful antioxidant. 

NAC reduces levels of pro-inflammatory cytokines and oxidative stress (source), and because it increases glutathione levels in the body, which are rapidly depleted by stress, drug toxicity, and disease, it has applications in clinical settings, in addition to general health and wellness. (Source)

NAC has been used for the treatment of cystic fibrosis (due to its mucolytic properties), acute acetaminophen toxicity(source), and chronic obstructive lung disease (source).

NAC has cytoprotective effects, (Cytoprotection is a process by which chemical compounds provide protection to cells against harmful agents), which work through antioxidant and anti-inflammatory mechanisms. On the antioxidant side, although the NAC molecule itself has the ability to scavenge reactive oxygen species (ROS), the antioxidant effects of NAC in the body mainly occur through increasing glutathione levels and other indirect mechanisms.

 The anti-inflammatory effects of NAC are indirect and work through its antioxidant activity, which in turn inhibits the pro-inflammatory transcription factor NFkB, which  regulates multiple aspects of innate and adaptive immune functions and serves as a pivotal mediator of inflammatory responses.