Pinging refers to sending auto-generated notifications to search engine spiders telling them that the page has been updated and needs to be re-crawled.
In SEO, pinging means to tell search engines about your newly created backlinks so that they are crawled and are counted. Else you will not go to get any benefit from them. Ping submission is helpful for crawling the website and for updating the information in search engines.

The main purpose of pinging your backlinks is to get them indexed faster by the search engines your target, so they can pass link equity faster than they would anyway.