"R" stands for "Restricted-Viewing."

Once upon a time there were some puritanical fanatics in the U.S. of A. that were getting really uptight about the content of movies (sound familiar?), and in response, the big-shots in the industry came up with an organization (later forming into the MPAA---the Motion Picture Association of America) that would act as a moral censor rather than the government.

Over time, the rating system emerged:
G--General audience; all audiences admitted
PG--Parental Guidance suggested; some material may not be suitable for children
PG-13--Parental Guidance; some material may be inappropriate for children under the age of thirteen
R--Restricted; under the age of seventeen requires an accompanying parent or guardian
NC-17--no one under the age of seventeen admitted (yeah right).