Pinging is actually the action where a certain aggregator is notified whenever your web log or blog is updated, so that the aggregator may show the freshest post of your blog on their website. This is also a very useful method to pulling in some traffic to your site.
Sending out a ping is like raising your hand in class to announce you have something to say. Your teachers, the search engines and aggegators are waiting to hear you speak. If you don’t ping the weblog tracking sites to let them know that your site has been updated, how will they ever know. The bigger your ping list, the higher the chances of receiving traffic from those sources.