Protected questions have the additional restriction that users with low reputation are not permitted to answer the question.
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.
For more information, see: