Campden tablets (potassium or sodium metabisulfite) are a sulfur-based product that is used primarily to sterilize wine, cider and in beer making to kill bacteria and to inhibit the growth of most wild yeast: this product is also used to eliminate both free chlorine and the more stable form, chloramine, from water ...
this is a snipit from the wikki page,
for me these are mainly for protecting my wort during a potential lagging phase. ill only use half a tab for 20 litres though. a whole tablet is for a 40litre batch (although i have put a whole one in at times)
i drop it in the fv and pour the wort in, im just hoping im not killing the tablet off in the process (if the wort is on the hot side).
Theyre also used as a substitute for a sanitizer but i think this is more an australian/New Zealand thing going by a couple of youtube vids
