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 (110 if they have the association bonus).
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 (110 if they have the association bonus).
Questions are typically protected to deter spam answers or noisy answers from new users who may mistake the site as a traditional forum on popular questions.
Questions can also be automatically protected by the Community user if they receive too many spam answers, answers from new users that get deleted, or answers from new users posted within a short period of time.