Apple cider vinegar is made in a two-step process, related to how alcohol is made (1). The first step exposes crushed apples (or apple cider) to yeast, which ferment the sugars and turn them into alcohol. In the second step, bacteria are added to the alcohol solution, which further ferment the alcohol and turn it into acetic acid — the main active compound in vinegar. In French, the word «vinegar» actually means «sour wine.» 

Apple cider vinegar is made by fermenting the sugar from apples. This turns them into acetic acid, which is the active ingredient in vinegar.

Organic, unfiltered apple cider vinegar (like Bragg’s) also contains «mother,» strands of proteins, enzymes and friendly bacteria that give the product a murky appearance. Some people believe that the «mother» is responsible for most of the health benefits, although there are currently no studies to support this. Apple cider vinegar only contains about three calories per tablespoon, which is very low. There are not many vitamins or minerals in it, but it does contain a small amount of potassium. Quality apple cider vinegar also contains some amino acids and antioxidants.

Source: healthline.com