In nuclear power plants, moderators serve to slow down the speed of movement (kinetic energy) of free neutrons.
Free neutrons are produced during nuclear fission. These free neutrons in turn maintain this fission process by splitting new nuclei. Each nuclear fission produces about two to three neutrons with a high speed of motion, so-called fast neutrons. Since fast neutrons are less likely to trigger nuclear fission than slow neutrons (so-called thermal neutrons), their speed must be slowed down by the so-called moderator.
Hydrogen and carbon are particularly suitable moderators. Nowadays, water is used as moderator in the vast majority of reactors, for example in light and heavy water reactors.