An OpenID identifier is a URL that represents an identity on the Internet. OpenID was invented by LiveJournal as a way for users to prove their control over a particular blog. It has since come to be used by Google, AOL, Yahoo!, Stack Exchange, and other web sites as a decentralized authentication platform to let someone with an account on one site (the "provider") log in to other sites ("relying parties"). Sometimes the identifier is specific to a user, such as a user's profile page or blog, and sometimes it's a generic identifier that the provider redirects to a user's identifier. For more information about OpenID, check FAQ for Stack Exchange sites.
As Bart pointed out in a comment to the question, Google is an OpenID provider. This generic identifier will trigger the Google Account login flow on Stack Exchange or any other relying party:
In any case, there are good reasons to access Stack Exchange sites with Lynx or another user agent that does not require a running GUI, such as doing things that the Stack Exchange sites support but which currently do not have a corresponding API function. Posting questions and answers used to be one of them back in the first quarter of 2011 when the API was read-only. As of the fourth quarter of 2014, one thing that the API still does not support is suggesting edits.
https://www.google.com/accounts/o8/idif I'm not mistaken