Autoconfirmed is a user group that is automatically given to registered user accounts that are more than 4 days old and have made at least 10 changes. It allows users to move pages to a different title and make changes to pages that have been semi-protected by administrators.
The main reason for having autoconfirmed status on Wikipedia is to prevent vandalism and other types of disruptive changing to popular pages by allowing only users who have shown they can make only good changes to change these pages. It can only be removed if the user triggers the change filters.
In some situations, it is necessary for accounts to be exempted from the customary confirmation period and instead confirmed right away. The 'confirmed' group contains the exact same rights as the 'autoconfirmed' pseudo-group, but can be granted by administrators as necessary. It is redundant to grant the confirmed right to an account that is already autoconfirmed, since it provides the same abilities.