Autoconfirmed users are wiki users who are members of the "autoconfirmed" user group. This is a special user group which is allocated automatically based on certain criteria.
The criteria are
- $wgAutoConfirmAge (number of seconds account needs to have existed to qualify)
- $wgAutoConfirmCount (number of edits account needs to have to qualify)
Note that BOTH criteria need to be satisfied for an account to be automatically promoted to the autoconfirmed user group.
What rights autoconfirmed users have is controlled via $wgGroupPermissions.
See also 
- Setting user rights in MediaWiki
- $wgAutopromote, for creating user groups with other automatic promotion criteria
- m:Registered user, m:Newly registered user