Control rods also displace moderator, so neutrons have lower probability of losing enough energy to initiate fission before they escape the reactor. You have chain reaction if each fission produces exactly 1.0000 neutron that initiates next fission. read more
Some isotopes' nuclei though do a better job of sucking up neutrons that pass nearby. It's important in the nuclear industry to have neutron absorbers. To control a reactor's power at a constant level, the amount of neutrons that produce fission in one second has to equal the amount of neutrons that will produce fission in the next second. read more