A ping is a short message sent automatically from your blog to another application. By pinging third-party applications, they will be aware of new content and those applications will then be able to retrieve that new content for their index.
Some ping services notify a variety of applications and web services--announcing that your blog, website, RSS feed, or podcast has recently been updated or created.
Ping services for blogs are essential for real-time news and breaking news stories. These services cut down the wait time for your content to be distributed to Google, blog search services and websites like Technorati.
Word of caution: Using a ping service does not mean your website will rank high on Google or other search engines. There is no magic bullet for SEO. Search engine optimization takes time and great content. Ping services are merely a way to notify Google and search engines that your content is available.