Depending on the context, the letter R can stand for several things. From our friends teaching organic chemistry at UCLA:
R group: An abbreviation for any group in which a carbon or hydrogen atom is attached to the rest of the molecule. Sometimes used more loosely, to include other elements such as halogens, oxygen, or nitrogen. R is an abbreviation for radical, when the term radical applied to a portion of a complete molecule (not necessarily a free radical), such as a methyl group. Should not be confused with R (the gas constant), R (the one-letter abbreviation for the amino acid arginine) or R (a designation of absolute configuration).
In the image from the Wikipedia article you have referenced, the letter R is as defined in the first section of the above quote - that is, it stands for "any group in which a carbon or hydrogen atom is attached to the rest of the molecule."