RT is the abbreviation for “ReTweet” which means to replicate something that has been written. RT on twitter means that the person copied, repeated the text that someone else had already written, but showing the proper credits.
RT on twitter usually happens when a person writes a sentence about something that is interesting to others, or when it's a matter of public utility, something that should be passed on to front. When the person RTs on twitter, the name and phrase they copied from the other person appears to others.
A message on twitter is called a tweet, and RT on twitter can happen to someone who the individual follow, or even friends of friends, just the person thinks it's an interesting message, to give a RT. re-tweet, in other words, it is a message that we receive from someone we follow and that we send (forward) to our followers (followers).
The RT on twitter can be done in two ways, one using the website link itself, where it says “Retwitter” or simply copying the phrase and putting RT in front, to be able to comment on it.
Learn more about Twitter.