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, provided the question has received at least one answer from a user with less than 10 reputation (such users would then be prevented from posting further answers after the question is protected).