Heavy water plays an important role in the functioning of nuclear reactors that use uranium which has not undergone enrichment to increase the percentage of uranium-235 as the fuel.
Uranium consists of many isotopes, the largest percentage being U-238 followed by U-235. Only atoms of U-235 can undergo fission. The fission of U-235 is initially started by bombarding it with neutrons. When atoms of U-235 breakup it generates more neutrons which lead to the fission of more atoms of U-235.
The role of heavy water here is to reduce the speed of neutrons that are produced. This is important as it increases the probability that the neutrons will react with U-235 and lead to more fission reactions rather than get absorbed by U-238 which would not cause any reaction. As heavy water has the isotope deuterium instead of normal hydrogen it also does not absorb neutrons like normal water does.
Heavy water due to these properties is essential to ensure that a chain reaction is started when normal uranium is used as fuel. The alternative to using heavy water would be to enrich the uranium and increase the percentage of U-235 which is an expensive process.
Heavy water is used as a moderator in nuclear reactors. It is used to slow the neutrons being directed at the fissionable material, by means of the molecules of the moderator physically impacting the incoming neutrons and absorbing some of the kenetic energy posses, thus slowing them down.