"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).