I presume it reads f(x) equals 0.4 times X plus 0.0002 times X cubed, etc? Where X is the roll command input? This equation is what generates the roll command curve the aircraft's control surfaces actually use?
Yep, that's correct, although to be more clear, f(x) is read as "f of x" (i.e. function of x). The input for x is expected to be in the range [-100,100] (i.e. negative 100 to positive 100, inclusive), representing the roll axis of the controller. The output of the function returns a value that's used as the set point for the PID controller to determine the desired roll rate of the aircraft.