In a nuclear reactor, the moderator slows down the neutrons so as to control the nuclear chain reaction and avoid an atomic explosion.
Principle of the nuclear moderator
A good moderator such as graphite, heavy water (deuterium) or normal water (hydrogen), slows the neutrons by elastic collision with the atoms, without absorbing them.
Ideally, the mass of the nuclei in the moderator should be as close as possible to the mass of the neutrons.
In a pressurised water reactor the water plays the role of moderator. © Steffen Kuntoff, Translation of Enr-v, Wikimedia CC by 1.0
Nuclear moderator - 1 Photo