These are basically sodium / potassium metabisulphite in tablet form. They can be used to sterilise wine and beer just before to bottling.
Six tablets dissolved in a pint of water can be used as a sterilant to rinse almost everything, especially bottles prior to filling.
Campden tablets are also used towards the end of the fermentation process to halt the ferment before all the available sugars are converted by the yeast, hence controlling the amount of residual sweetness in the final product. This balancing between sweet, dry and tart flavors is part of the artistry of wine and cider making.
Campden tablets typically weigh 0.44 g each and 10 of these are equivalent to one level teaspoon of sodium metabisulphite.
Avoid prolonged contact with stainless steel, as they will attack the surface. Asthmatics should use caution as exposure to the fumes can aggrevate asthma.