From Halopedia, the Halo wiki
Nuclear fusion is a process where multiple atomic nuclei join together to form a heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy. The process powers active stars, hydrogen bombs and fusion power plants used for electricity generation.
Aside from employing traditional fission-based nuclear power, humanity has mastered nuclear fusion as a viable source of energy. Among other uses, it is employed in the fusion reactors that power starships' propulsion systems and reactors used to provide power for prefabricated base units. Fusion reactors are fueled by reactions between isotopes of hydrogen, such as deuterium.
The Covenant use pinch fusion reactors to power their starships and weapon emplacements. These reactors employ tritium-deuterium reactions and pinched magnetic fields that compress and heat the plasma produced by the reaction.