The mouseover text says "Report post to moderator", and that's exactly what is does: alert the forum moderators to a post that needs administrative attention. This could be the case if a post contains spam or other inappropriate content, is abusive, contains links to copyrighted material, etc. In other words, all the stuff that might lead to a moderator editing a post, or otherwise take action.
Sometimes the forum moderators might not notice this on their own, and quicker action can be gotten by alerting a wider range of us to it.
It is NOT meant to draw attention to a post along the lines of "my question has not been answered for N days, please take a look" - that is not the role of the moderators. We'd put a stop to that real quick