Passage: Most fission reactors are thermal-neutron reactors that use a neutron moderator to slow down ("thermalize") the neutrons produced by nuclear fission. Moderation substantially increases the fission cross section for fissile nuclei such as uranium-235 or plutonium-239. In addition, uranium-238 has a much lower capture cross section for thermal neutrons, allowing more neutrons to cause fission of fissile nuclei and propagate the chain reaction, rather than being captured by 238U. The combination of these effects allows light water reactors to use low-enriched uranium. Heavy water reactors and graphite-moderated reactors can even use natural uranium as these moderators have much lower neutron capture cross sections than light water.An increase in fuel temperature also raises U-238's thermal neutron absorption by Doppler broadening, providing negative feedback to help control the reactor. Also, when the moderator is also a circulating coolant (light water or heavy water), boiling of the coolant will reduce the moderator density and provide negative feedback (a negative void coefficient).

Question: What us the meaning of thermalize?
thermal-neutron reactors that use a neutron moderator to slow down