The second initial in J.R.R. Tolkien’s name stands for Ronald.
His full name was John Ronald Reuel Tolkien. He was born John Ronald Reuel Tolkien on 3 January 1892 in Bloemfontein, and “Ronald” was used as one of his given names throughout his life. The final initial, R, stands for Reuel.
The name Reuel came from Tolkien’s family history and was also used as a middle name for his children. Tolkien is normally identified publicly by his initials rather than by his full given names, making the expansion of those initials a frequently asked biographical detail.
The second R is not a separate middle initial for a different first name. Both R letters represent parts of his given-name sequence: Ronald and Reuel. This distinction makes “Ronald” the precise answer when the question asks about the second initial.