Every post can be "voted up" or "voted down". That way users accumulate "good karma" or "bad karma". People with better karma can assign or subtract greater number of points from others' posts. If a post is very low-quality and gets lot of negative downvotes, it automatically becomes hidden, and a user has to specifically click the post to expand it and make it visible. That's how it works in brief - there are a lot of potential innovative enhancements possible to this.