Sorry for poorly commented code...
"Xstd_rgb" means standard deviation of observation noise, which means noise you get when you observe the state of something.
"Xstd_pos" and "Xstd_vec" mean standard deviation of system noise, which describes how far actual movement of target object differs from the ideal model (in this case, linear uniform motion).
State space become of 2 componets, one is "position of particle" and another is "speed of particle".
You can define these 3 types of noise by these parameters.