What is N-Acetyl L-Cysteine?
N-Acetyl L-Cysteine (NAC) is an acetylated form of cysteine, an amino acid. NAC converts into cysteine which helps synthesize glutathione in the body. Both cysteine and glutathione have been explored as stand-alone supplements. However, both supplements are broken down too fast to be of much use hence the preference of NAC.  Glutathione is a potent cellular healthy supplement, capable of fighting free radicals.*  Free radicals damage cells and can contribute to cognitive decline.* 
N-Acetyl L-Cysteine Benefits
NAC has a number of benefits, including supplementing cellular, cognitive and cardiovascular health. Secondary to its cellular health properties, N-Acetyl L-Cysteine may be contributing to alleviating the damage caused by free radicals.*  NAC has also been shown to promote healthy mood. 
As mentioned above, NAC converts into L-cysteine within the body which supplements the amount of glutathione your body makes.* This is significant because glutathione is able to scavenge free radicals and limit their deleterious effects.  Even so, glutathione degrades over time. It is limited by the amount of L-cysteine present.
Since direct supplementation with L-cysteine is ineffective, N-Acetyl L-Cysteine supplements are the preferred alternative. NAC converts into usable L-cysteine, which then promotes the glutathione resynthesis process.*  NAC is effective at scavenging hydroxyl radicals and hypochlorous acid.  Once converted into glutathione, it can help detoxify reactive oxygen species at the cellular level. 
N-Acetyl Cysteine Dosage
The recommended NAC dosage is 500mg per day.