R- and S- notation is used for the assignment of the absolute configuration around a stereocenter.

First, assign priority to each atom surrounding the stereocenter.

Priority is based on atomic number.

Second, point the lowest priority atom away from you. Follow the direction of the remaining 3 priorities from lowest to highest.

A counterclockwise direction is an S  configuration. A clockwise direction is an R  configuration.

Direction of the travel 1-2-3 dictates configuration