What does RT mean on twitter?
Simple answer: Re-Tweet

It is used in two different ways. Below we show by recent examples two uses of RT.

1. To give credit to the person who originally tweeted something.

In this Tweet, National Museum are Re-Tweeting a Tweet from @sophiechoudry.
So the Formula is

RT Person who Originally Tweeted :  Original Tweet

Here
Person who Originally Tweeted : @sophiechoudry
Original Tweet: The huge #Hobbit banner  is up @mediamuseum instagr.am/p/SSSKCyqplh/

2. To ask for a Re-Tweet

Here Welcome to Yorkshire are asking people to Re-Tweet this Tweet.