Control Rods absorb thermal neutrons when inserted into the core (through spaces specifically built into the fuel assemblies). GE BWRs insert the control "blades" upwardly from the bottom of the reactor. This requires hydraulic actuators known as Control Rod Drives. PWRs use gravity to rapidly insert the control rods (when released) from above.
At Fukushima, the control blades inserted, killing the fission reaction. Unfortunately, decay heat removal failed and resulted in the devastation we are now left with.