What are the R_left and R_right in stereo rectifier parameters?

[Ab-Tx]

Hello, I am configuring stella_vslam for the ntu viral dataset.

When configuring a stereo setup the R_left and R_right can be specified, but what are these exactly?
From the other datasets configurations I would guess these are rotation matrices, but I'm unsure as the documentation refers to it as "Stereo-recitification parameters".

Also, how can this parameters be calculated?

[Ab-Tx]

https://docs.opencv.org/3.4/da/d54/group__imgproc__transform.html#ga7dfb72c9cf9780a347fbe3d1c47e5d5a