Here is the entirety of the help center page of the privilege for protected questions:
Privilege Type: Moderation Privilege Awarded At: 15,000 Reputation
What are protected questions?
A protected question prevents answers being added by anonymous and very new users.
Questions should be protected when they are garnering lots of views and newbies are adding "me too!", "thanks!" and possibly even spam non-answers.
Which questions can be protected?
Any question at least a day old can be protected and unprotected by users that have the privilege.
Notice the absence of the link to the faq post, which has a lot more information.
Related (this is the problem I am trying to address): Improve "Protected Question" message with regard to rep gained through Association Bonus