From the Help pages:
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).
For more information see the blog post New Protected Question Status.
Some additional comments:
Sometimes the reasons for protecting a question are limited to a specific time window. This happens most often when questions get promoted to the Hot Network Questions sidebar, which gets advertised across all 150+ Stack Exchange sites, and that normally brings in a large influx of association-bonus users who have a minimal involvement in this site and its community, but who can upvote (but not downvote) questions and answers (leading to a pretty pernicious feedback loop), and who are quite often happy to add in some rather off-topic answers. Protecting those questions is a very welcome tool to keep the discussion on-topic, and (like a bunch of other janitorial tasks, like tag edits or helpful "related: ..." comments) very often it is Qmechanic that gets there first, as discussed in this previous thread.
If the reasons for protecting a question are no longer valid, then it is perfectly reasonable to raise a custom flag on the question (or to pop into the site chatroom, if there are >15k-rep users around) asking whether it should still be protected. If the reasons are no longer present, the response will often be to unprotect the question.
On the other hand, some questions have long-running reasons for protection, but those reasons will not be visible unless you have >10k rep and you can see all the deleted non-answer answers that caused the protection in the first place. For an example, see the blog post linked above; for something on-site, try e.g. this question: it might look perfectly reasonable to you, but it has eight deleted (terrible!) non-answers spread out over one year, and the protection is clearly appropriate.
If a question gets protected, the only effect is that posting an answer requires you to have earned 10 rep on this site. This is an exceptionally low threshold (one upvote on an answer, or two on a question), and the body of people with meaningful answers to add that get a participation barrier for them to post it is (while nonzero) small enough that the protection against spam and junk answers is justified. Or, to put it another way: the banner can look big and scary, but its real-world negative impact are generally extremely limited. (On the other hand, there is a selection bias in that if people are turned away, we're unlikely to hear about it. But still.)
Generally, we're not great at unprotecting questions that no longer need it, which is partly because there isn't a good volunteer base to identify them, and partly because the existing software doesn't have great tools to find those questions. (One such tool might be e.g. a review queue of questions that stopped being HNQs three months ago, which are probably ready for unprotection. That's unlikely to be implemented by SE, though.)
However, there's nothing stopping a motivated-enough volunteer from looking through old questions and flagging them (/ asking 15k'ers on chat) to unprotect them. The 10k tools have a dedicated stats page with protected questions, but all that data is public and e.g. this Protected Questions SEDE query is an excellent starting point. (If you do go there, though, and you've got <10k rep, be wary of questions with multiple deleted answers, as reported in the query, that you cannot see.)