I notice you’ve asked a number of similar, basic questions @Mehwish. Before posting, it’s always a good idea to search the forums, as you’ll often find such questions have been asked and answered before. So you can get your answer straight away, without having to wait for somebody to reply.
There are also numerous articles on the wider Internet. I suggest you try these (or search for others) and if there is something unclear, then feel free to ask about it here.
http://www.seostrategies.pro/seo/369/pinging-for-seo-explained/