Nuclear Power - Controlling the energy output of the reactor

Boron is a very good absorber of neutrons. It is said to have 'a high cross section for neutrons'.

Rods of boron (or cadmium - because it has a high cross section for neutrons too) are lowered into the reaction-vessel. They are lowered deep enough to absorb a sufficient quantity of the neutrons produced by fission so that only about one of those produced in a split goes on to instigate a new fission. If more energy is required by the power station, the rods can be lifted out of the vessel (allowing more neutrons to cause fission). If less energy is required they can be lowered deep into the vessel (mopping up more neutrons). If they are pushed right in, they can mop up enough neutrons to prevent the chain reaction from continuing; this will result in shutting down the reactor safely.

These rods are called control rods because they control the energy output of the reactor.