User blog comment:BananaRamba/Auto-confirmed User?/@comment-24808864-20151216172401

In order to protect against spamming vandals (a person who would make multiple accounts and vandalize pages with all of them) some pages can be protected so that they can only be edited by "Auto-confirmed" users.

The designation occurs automatically after some amount of valid edits and/or days on the wiki which is why it is called "auto"-confirmed. This designation comes automatically.

According to Balu below and Tibet in another thread, the threshold is 10 edits/4 days.