Power output is directly proportional to the rate of fission reactions. When a reactor is critical, it has a sustained power output. When the reactor is supercritical, the power output is increasing, and when it is subcritical, power decreases until the reactor shuts down.
Reactivity is way of describing the criticality condition of the reactor.
Positive reactivity means a supercritical reactor, zero reactivity means a critical reactor, and negative reactivity means a subcritical reactor. The rate of fission in a reactor is controlled through the insertion and withdrawal of neutron-absorbing material, such as boron, in the form of control rods which are interspersed with the fuel rods. By lowering and raising the control rods, which absorb neutrons available for fission reactions, the rate of fission reactions, and hence the reactor's power output, can be controlled. A particular property of fission makes it possible to achieve reactor control. These are known as delayed neutrons.