The “R” is the pattern’s root. You can use the root to determine where to play the pattern in other key signatures. The pattern’s root note in the illustration is on the A note on the 5th fret of the low E string and therefore in the key of A. If we wanted to play the pattern in the key of B we would play the pattern to whereas the root note is on the B note on the 7th fret of the 6th string. In other words, we would just move the entire pattern up two frets. If we wanted to play the pattern in the key of C we would move the root to the C note on the 8th fret and apply the same logic to find the correct positioning of the pattern by its root note for all other key signatures.