An Autoconfirmed user, on many wiki-based sites, is a contributor who may have various technical privileges over those of ordinary users. 'Autoconfirmed' status is typically imposed automatically by the MediaWiki software once certain thresholds of activity have been met, e.g. the user would have to have made a certain number of edits, and/or have created the account a certain length of time ago. This can allow staff to identify recent arrivals, for example.
On Citizendium, the status is awarded automatically when new accounts are created, so there is no special group of autoconfirmed users - everyone has it. Though being 'autoconfirmed' permits the editing of semi-protected pages, semi-protection is not currently used on the site, so the status carries no practical privileges.
See also the user groups rights page, which lists other user groups.