"RT" or "retweeting" is simply taking a twitter post from someone else and forwarding (rebroadcasting) it to your followers. Here are a few common ways to retweet a message (all do the same thing):
RT @originalsender: original messageRetweeting can be a great way to add followers, as it pushes your @username into foreign social graphs, which in turn results in clicks back to your profile. You can also track your retweets using retweetist.
retweet @originalsender: original message
retweeting @originalsender: original message
To retweet this post, type:
retweet @techack: Retweeting explained: http://tinyurl.com/retweetexpP.S. You can join me on Twitter at http://twitter.com/techack