What "R" really means.

You may simply think of "R" being the Rest or Remainder of an amino acid; the position at which the twenty naturally occurring amino acids differ from each other. To be an amino acid, a molecule must have an amino group (NH2)and an organic acid (carboxyl) (COOH) group. The portion of the amino acid where they differ is a central group of atoms -- the "R" group. The "R" may be as simple as a single hydrogen atom (H) or a methyl group (CH₃) or the "R" may be a complex group of atoms that is itself larger than the rest of that amino acid.

The chemical designation for the "R" group is that it stands for a radical. A radical is a functional group of atoms bonded together.