Again, this means that only the person who commented, and their friends, can see the post.
Now, you may be thinking "Why not simply block these trolls? Or delete their comments?"
There are a couple of reasons. First - if you delete comments then you set yourself up to receive even more irate criticisms about freedom of speech and so on. Similarly, if you block trolls then you'll probably receive messages from them wanting to know why. This can cause more admin work for you, and provide trolls with more fuel for their hostility.
By hiding comments, the trolls don't know that their post is hidden - and even troll comments add to your engagement factor. So every hidden troll comment increases the number of times Facebook displays your post, which ultimately increases your reach and contributes to increasing your social media followers.