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: 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