Ping, in the simplest terms, is an Internet tool that knocks on the doors of search engines and directories to invite them to crawl your website or blog.
When you publish a post on your blog, modern blogging programs send a signal to a pinging service such as Pingomatic which relays a digital invitation to seach engines, tagging services, and other web crawling services to visit your blog.
If you wish to manually ping, do it more than once within a reasonable length of time can result in penalization. Pinging is like submission to a search engine and carries penalization for abuse.
Manual pinging is a great way to reach search engines and directories outside of the most popular indexing services.