what does a moderator do in a nuclear reactor

a moderator is a medium that reduces the speed of fast neutron thereby turning them into slow neutron ,their main function is to reduce the speed of fast neutron and therefore slow down the entire reaction, they do this by reducing the kinetic energy of the free neutron , therefore when the neutron collide with the atoms of the moderators they lost their energy when the collision is inelastic.

  light water, solid graphite and deuterium water are the most common type of nuclear moderators