To retweet a message in Twitter, all you have to do is to simply click the "RETWEET" button on the upper-right portion of the (tweet) message you want to RT.
RT is the short form of Re Tweet. With this feature,you can forward a tweet by any other person to your followers.Its like forwarding an email or a message.
People retweet or use RT to share a tweet to their followers. It is also a sign that the person like or agree the message, it's actually a counterpart of facebook's LIKE button.