Serial numbers in DNS zone files provide a way for the server to verify that the contents of a particular zone file are up-to-date. If the serial number in a zone file hasn’t changed since that zone was last loaded, named figures that it can ignore the file.
Chrome Users: Try the WordPress.com Extension — Blog — WordPress.com. Got Chrome? Here’s more polish.
Convenient is the first word that comes to mind, along with current, timely and relevant.