a nuclear chain reaction

Fission is useful in bulk only because it feeds itself. Each fission of a heavy nucleus not only releases energy but also throws out two or three free neutrons, and those neutrons can strike other fissile nuclei and split them in turn, each of which releases more neutrons. A self-propagating cascade like this is a nuclear chain reaction, the multiplying process at the heart of every reactor and fission weapon.

Whether the cascade grows, holds steady, or dies is captured by the multiplication factor k, the average number of new fissions caused by the neutrons from one fission. If k = 1 the reaction is critical and self-sustaining at constant power (a running reactor); if k > 1 it is supercritical and the rate grows exponentially (weapon, or a reactor increasing power); if k < 1 it is subcritical and fades away. Achieving k >= 1 requires enough fissile material arranged so that too few neutrons leak out or are wastefully absorbed: hence a critical mass, and in reactors a moderator to slow neutrons to fission-friendly thermal energies, plus fuel of sufficient enrichment.

The reason reactors are controllable at all is a subtle gift of nuclear physics: a small fraction (under one percent) of fission neutrons are delayed neutrons, emitted seconds later by the beta-decay of certain fission fragments rather than promptly. Reactors are run so that they are subcritical on prompt neutrons alone and reach criticality only counting the delayed ones, which stretches the response time from microseconds to seconds and makes mechanical control rods able to keep up. Without delayed neutrons, controlled fission power would be effectively impossible.