Complex sentence: Most fission reactors are thermal reactors that use a neutron moderator to slow down, or "thermalize" the neutrons produced by nuclear fission, which substantially increases the fission cross section for fissile nuclei such as uranium-235 or plutonium-239.
Split into simpler sentences: Most fission reactors are thermal reactors that use a neutron moderator to slow down, or "thermalize" the neutrons produced by nuclear fission. Moderation substantially increases the fission cross section for fissile nuclei such as uranium-235 or plutonium-239.