To "retweet" is to repeat/quote someone's tweet. Usually when you come across and interesting tweet and you want to publish it as your own tweet so that people who follow you see it too - you retweet it.The ridiculous things I have to learn at my age. I thought RT stood for "Reply To" until this morning.
The syntax of your tweet should start with the abbreviation RT or the word Retweet followed by the username of the person who tweeted it (e.g @User) and then finish with the content of the actual tweet.
Oscar Madison: "We're all out of cornflakes. F.U." Took me three hours to figure out F.U. was Felix Ungar!