It's an Over the counter supplement. It breaks up the chains of mucus it easier for your body to clear it out.
From wikipedia:
Acetylcysteine, also known as N-acetylcysteine (NAC), is a medication that is used to treat paracetamol (acetaminophen) overdose, and to loosen thick mucus in individuals with chronic bronchopulmonary disorders like pneumonia and bronchitis.[2] It can be taken intravenously, by mouth, or inhaled as a mist.[2] Some people use it as a dietary supplement.[6][7]
Common side effects include nausea and vomiting when taken by mouth.[2] The skin may occasionally become red and itchy with any route of administration.[2] A non-immune type of anaphylaxis may also occur.[2] It appears to be safe in pregnancy.[2] For paracetamol overdose, it works by increasing the level of glutathione, an antioxidant that can neutralise the toxic breakdown products of paracetamol.[2] When inhaled, it acts as a mucolytic by decreasing the thickness of mucus.[8]
Acetylcysteine was initially patented in 1960 and came into medical use in 1968.[9][10] It is on the World Health Organization's List of Essential Medicines.[11] It is available as a generic medication and is inexpensive.[12]