From the Manufacturer:
NAC (N-acetyl-L-cysteine) is an antioxidant that helps increase glutathione synthesis, and potentially has benefit during oxidative stress. NAC is the acetyl derivative of L-cysteine. While L-cysteine plays important metabolic roles as a key antioxidant, a glutathione precursor, and a natural source of sulfur for metabolism, it is unstable and can become degraded during absorption. NAC on the other hand, is more stable than L-cysteine. Taken orally, NAC converts into L-cysteine after being absorbed, and raises blood and tissue cysteine levels. As a dietary supplement, NAC is a highly attractive alternative to L-cysteine.