Problem of MPC's expected trajectory

[YuXianYuan]

Hello, everyone,
In the file“ ConvexMPCLocomotion.cpp "We can find this code:
float trajInitial[12] = {(float)rpy_ comp[0],
(float)rpy_ comp[1], // 1
...
In the paper "dynamic locomotion in the MIT chetah 3 through converse model predictive control", they set the expected pitch and roll to 0, but what is "rpy_ comp" here?

What do they mean by the "Integral-esque pitche and roll compensation" method they use?