You can see a description of the FM signal here:
https://en.wikipedia.org/wiki/FM_stereo#Stereo_FM
L is short for LEFT channel and R is short for RIGHT channel. The FM Signal is broadcast in mono and the subcarrier (38 Khz) carries the L-R information. The 19 Khz pilot tone allows the detection of the stereo broadcast and regneration of the 38 Khz Carrier which is 2x19 KHz.
Adding and subtracting the two equations L+R and L-R yields 2L and 2R meaning 2x the amplitude.