It's a stupid button that I would remove if I could. I think it's meant to be used for punishing trolls for making hurtful/racist comments and the like...which should really be the jobs of moderators. But it sometimes gets used as a "dislike" button, which in other forums is often used by someone AGREEING with you about disliking something. Here, though, it permanently subtracts one point of rep from the original poster, so it's kind of more like a "I dislike YOU" button.