Just to directly answer your question... logged-in users can flag submissions using the "flag" link under them, or flag comments by clicking on their permalink

Just to directly answer your question... logged-in users can flag submissions using the "flag" link under them, or flag comments by clicking on their permalink (the relative timestamp) and then flagging on the new page that opens. The implication is that probably another user flagged your submission because they considered it off-topic. Generally anything political will immediately get flagged because people here don't like to discuss that.