Question

state what the R shown in the amino acid diagram refers to.

Answer 1

R refer to the absolute configuration. In this configuration the substituents are each assigned a priority. The priority is after the substituents weight (in some most cases). When this is done we look at the 3 highest priority and determine about we are going to turn the molecule in a clockwise direction (R (Rectus, Latin for right)) or counterclockwise direction (S (Sinister, Latin for left))

Answer 2

The question is probably referring to this R: