The multiplication factor $(k)$ is an important reactor parameter and is the ratio of number of neutrons present at the beginning of a particular generation to the number present at the beginning of the next generation. It is a measure of the growth rate of the neutrons in the reactor. For $k=1$, the operation of the reactor is said to be critical.
Note: If $k$ becomes greater than one, the reaction rate and the reactor power increase exponentially. Unless the factor $k$ is brought down very close to unity, the reactor will become supercritical and can even explode.