The moderator is a material which slows down the speed of the neutrons. The moderator is usually made from water but some reactors use graphite also.Neutrons released from the chain reaction must be moving properly for other atoms of uranium to capture them.If the neutrons are moving too fast then they do not get captured by the nucleus of another uranium atom and the chain reaction stops. The moderator slows the neutrons down so that they can be captured and keep the chain reaction going.