ETS, the Educational Testing Service, in charge of the SAT, AP exams, etc. had a problem. The scores received were very low for the majority of students.
The solution was to introduce the grade curve to the SATs. The 'R' means that your score has been subjected to this.
First, they find your raw score for the SAT,= how many you got right - (# you got wrong * 0.25).
Then, the raw scores are taken and curved with the rest. The score you get is your scaled score. No matter what, you get a minumum score of 100 per section, ("You get 100 points for writing your name"). The highest score you can get is 800. The median score is 500 per section. Supposedly if you get a 500, you are close to 50 percentile, although I've seen as low as 43% for a 570. Every test has a certain amount of variance to the scaling. YMMV
This counts for the SAT I and SAT II.