From the Manufacturer:
N-acetyl Cysteine (or NAC) is a more stable form of the amino acid L-Cysteine. As a precursor to L-Cysteine, NAC and helps the body synthesize Glutathione. NAC possesses antioxidant properties that may help fight cell-damaging free radicals. Studies have shown that oxidative stress caused by free radicals may contribute to premature aging of cells.