I'm just going to shamelessly plagiarise the answer from here
What does it mean for a question to be protected?
Protected questions have the additional restriction that new users are not permitted to answer the question. Unlike locked questions, they can still be edited, commented on, and voted on. You can even vote to close them.
Why are some questions protected?
Some questions are protected because they are expected to attract either spam or users -- often new users -- who may mistake the site as a traditional forum, posting "noisy" answers such as "Thank you" or "This worked for me" or "I'm also having this problem".
Who can protect and unprotect questions?
Who can answer a protected question?
Users with 10 or more reputation can answer a protected question. However, the +100 account association bonus is ignored for this check, so you must have earned 10 or more reputation on that specific site to answer a protected question.
When should I protect or unprotect a question?
- Do protect questions that are attracting a lot of non-answers or very poor answers (spam, etc.) from new users.
- Don’t protect questions just because they’re linked to on a high-traffic news site.
- Do unprotect questions that aren’t currently attracting a lot of attention and don’t have a long history of unproductive answers.