It is where different sites are told that your blog has been updated. It is useful to ping your blog about every other day, depending on how much you update the blog. If you use Wordpress, it will ping your blog after every post automatically.
Simply put, blog pinging is blog notification. It can notifying other blog search engines, other blogs, or even people.
Most wordpress blogs are already pinged, but if you want to do it manually: Also, this might be useful to you: http://www.maxblogpress.com/plugins/mpo/