"R" means "restricted" only to be viewed by adults, the reason being that this particular movie is either to violent or too sexually graphic for young people to watch.
It could be the nudity...or the male sex inuendos...or possibly all the swearing. It might be all the references to racism or incest. It could also be the drug use.
Zack Snyder (the director) tried to stay as faithful to the graphic novel as possible, and in doing so, included a lot of material from the comic that led to the movie being rated R, like excessive...