To turn on moderation, simply go to the gear icon next to your Social Feed and choose "Admin must approve." You must publish your app for the change to take effect.
When a non-admin next posts to that Social Feed page, an admin will get a push notification of the new post and can approve or reject the post.
The poster will see a pending state until the admin approves or rejects it.
When/if an admin accepts it, the post will appear as a regular post in the Social Feed.