Capping a tooth in UK, is normally done when the tooth is originally too decayed to just apply fillings. Normally the original tooth is cut down to good bone, virtually a stump, then a false tooth made of porecelan or gold, modeled to your original tooth is cemented onto this stump. Often a temporary standard false tooth is used, while your modelled false tooth manufactured. One problem that occurs, is the temporary one comes off easily. It is afterall not stuck on too hard, since your dentist has to remove it to afix your new false tooth. The other technique is to drill out the bad tooth, attach a peg, onto which the new false tooth is sited. This later method is very expensive & requires much dental time. My sister had ALL her's done this way some years ago.
(She was wealthy then! But felt, still does feel it was a good investment)

teeth capping ??what is this?? this is the first time i am hearing about this ..??can u just tell me what is that ??and what is the main use of teeth capping??

You should ask your doctor itself what will best suit your needs to be done.