It is a stereochemical label to indicate the relative spatial orientation of each atom in a molecule with a non-superimposable mirror image.
R indicates that a clockwise circular arrow that goes from higher priority to lower priority crosses over the lowest priority substituent and that lowest-priority substituent is in the back.
The R and S stereoisomers are non-superimposable mirror images, which means if you reflect them on a mirror plane, they do not become the exact same molecule when you overlay them.
When you label a molecule as R or S, you consider the priorities of each substituent on the chiral carbon (connected to four different functional groups).
Let's take this chiral amino acid for example:
Some general ways you could determine the priorities are:
- HIgher atomic number of the directly-attached atom gives higher priority
- Atomic number of the atom attached to the one is considered in step 1 if two substituents have the same first atom
- Higher number of same-atom branches determines greater priority if the overall substituents are too similar (e.g. isopropyl has higher priority than ethyl)
- #"NH"_2# has priority 1 due to highest atomic number for #"N"#.
- #"COOH"# has priority 2 due to the higher atomic number of #"O"# vs. #"H"# in #"CH"_3#
- #"CH"_3# has priority 3 as a result.
- #"H"# has priority 4.
Now, if you draw a circular arrow starting at #"NH"_2#, going to #"COOH"#, crossing over #"H"# since it is in the back, and to #"CH"_3#, then you would have gone clockwise.
Since the lowest priority atom is in the back, the clockwise arrow corresponds to the R configuration.
If you had started from the same R configuration but oriented #"H"# in the front and #"CH"_3# in the back, it would have been S configuration. Let's call this S configuration A, where you just nudge two substituents to flip them from front/back to back/front.
If you reflect the same R configuration over a mirror plane, keeping the orientations of #"H"# in the back and #"CH"_3# in the front after the flip, the configuration is also S. Let's call this S configuration B, where you've actually done a reflection.
If you start from S configuration B, and flipped it over a vertical axis (literally rotating #180^o# in space), you would get S configuration A.