The second R in J.R.R. Tolkien stands for Reuel.
His full name was John Ronald Reuel Tolkien. He was commonly known as Ronald within his family, while the initials J.R.R. became the form used on his books and in literary references. Reuel was also the name of his father, Arthur Reuel Tolkien’s middle name, and it had an unusual family history.
Tolkien’s parents chose the name partly because a German family friend named Reuel had been regarded as a particularly good and kind person. The name is also biblical in origin, appearing in the Hebrew Bible as a name associated with Moses’ father-in-law in some translations and traditions.
The author is often called simply Tolkien, especially in discussions of his books, but the initials distinguish him from other members of the Tolkien family. His son Christopher Tolkien, who edited many posthumous works, had a different full name and was not the source of the second R.