Campden tablets (potassium or sodium metabisulfite)[1] are a sulphur-based product that are used primarily to sterilize wine, cider and in beer making to kill bacteria and to inhibit the growth of most wild yeast.
Campden tablets allow the amateur brewer to easily measure small quantities of sodium metabisulfite, so they can be used to protect against wild yeast and bacteria without affecting flavour.
The sodium metabisulfite in the Campden tablets will trap oxygen that enters the wine, preventing it from doing any harm.
It is a common misconception that Campden tablets can be used to halt the ferment process in wine before all the available sugars are converted by the yeast, hence controlling the amount of residual sweetness in the final product.
Without the addition of potassium sorbate the yeast population will only be stunned and eventually repopulate if provided with enough fermentable sugars.