To paraphrase Wikipedia: in blogging, a ping is a mechanism by which a blog notifies a server that its content has been updated. A signal is sent from the blog to one or more "ping servers" who then notify a list of their "Services" about the new content available on the blog.
For example, if you sign up with ping server Ping-o-Matic, it will go ping other ping servers for you. What happens then, for example, is that those services then notify their many subscribers (and the search engines) that there is new content from your blog. Think of it like an intermittent "streaming" app that only streams when something new is found.
The whole purpose of pinging is to get more people to put eyeballs on your blog. It does not have any apparent impact on your SEO; it will not improve your ranking at Google. Nowadays, if you have a WordPress blog, you have many available ping plugins that will automatically ping for you whenever you update a post on your blog. Many bloggers don't bother with pinging, instead using one of many WP plugins that will post the blog entry on your social media pages, to let your followers on Facebook or Instagram or Twitter know there's something new to see.
For over a decade, we maintained a list of many ping servers on this page you are now reading. That service is being provided very well by others now, so these days we simply refer you to them:
Blog Ping List
- Social Compare - has an easy-to-read chart showing the pros and cons of each ping server. Go here first.
- Pingomatic will go and ping some other services on your behalf. Its short list includes Blo.gs, Feed Burner, Spinn3r, and Superfeedr.
- InfoBeat - top 10 sites to ping your blog! Note that it duplicates most of the info on the link above.
- Clever Plugins - a funny dissertation from Lars Koudal on whether pinging helps with SEO. (The short answer is "No!")