Campden tabs kill off any unwanted things (wild yeast, bacteria, etc...) that might come in on the fruit; it also helps to protect the wine/cider/mead/etc... from oxidation after the primary fermentation is complete.
Jack keller says it so much better than I can, have a look at this: http://winemaking.jackkeller.net/glossary.asp
just scroll down to the terms. You might have a look at the rest of his site too, it's a good one & will help to explain a lot of things. Hope this info helps. Regards, GF.