If the driver in question is over the age of 55, the "R" rating could stand for "retired" occupational status. To check, see if any other drivers on the policy have an "E" rating which may stand for "employed." However, if no drivers on the policy are retirement age, the "R" may simply stand for "rated."
Generally, all drivers listed on a policy will be rated drivers, but there are a few exceptions. Any household resident who is of driving age but is an excluded driver cannot be rated for insurance purposes as they serve no risk. For instance, consider a husband who has surrendered his driver's license due to a medical disability. Because his name is on the title to the car, he must have insurance on the vehicle. While his wife would be rated "R," since she is able to drive, the husband would be rated "NR," or Not Rated. So, if the "R" indicates that the driver has been rated, then all licensed drivers should have this notation.
Of course, you always have the option of asking your agent what the "R" stands for. You may also be able to sign onto your customer portal on your insurance website (if your insurer offers this feature) to see if there is any information there. Don't feel silly asking questions, regardless of how trivial they may seem. Accidents do happen occasionally and it's good to make sure everything on your policy is just how it should be.