These are chemicals used to kill bacteria in wine, and so act as a preservative. Metabisulphite powder can be used to sterilise bottles and brewing equipment; and Campden tablets (sodium or potassium metabisulphite) are used to stop fermentation before bottling – The sulphites given off when a crushed tablet is added to the wine will kill the yeast.
It is important to note that some people should avoid drinks containing any sulphites as they aggravate respiratory membranes, and no one should consume them alone.
In general, it is best to use sulphites sparingly. If you do use them for sterilisation, be sure to rinse everything thoroughly before using.