Questions may be protected by users with 15,000 reputation or by diamond moderators. Protected questions can only be answered by users with at least 10 reputation.
Questions may be protected by users with 15k reputation or by diamond moderators, the only effect of which is only users with at least 10 reputation may answer it.
Questions are typically protected to deter spam or "me too" answers on popular questions.
Questions can also be automatically changed to "protected" if three or more answers from low reputation users are deleted.