To use OpenID you must first establish an identity on a public or private OpenID server. If you do not have an OpenID and would like one, look into one of the free public providers. You can find out more about OpenID at this website.
If you already have an OpenID, enter the URL to your OpenID server in your profile settings page. Next time you login, you will be able to use this URL instead of a regular username and password. You can have multiple OpenID servers if you like.