You found a lot of users, who are putting RT before their message or tweets. RT is the short term of ReTweet, just like repeating.
In Email you might have forwarded interesting mail to your friend. So, it is very much common to forward email. Here in twitter, users forwards their favorite tweets to his/her followers instead of emails. Say a person whom you are following tweeted any interesting stuff and you find that tweet quite interesting. And you are wanting to send the same text on your tweet then you may use :
RT @originalcontentusername then the copied text. Its simple!
This is very useful to grow number of followers.
