Nuclear fusion releases energy when light nuclei combine into heavier ones. Achieving fusion on Earth requires extreme temperatures and sufficient confinement to keep plasma stable long enough for reactions to occur.
Two main approaches are magnetic confinement (such as tokamaks and stellarators) and inertial confinement (using lasers or particle beams). Each faces significant engineering hurdles.
Even with breakthroughs, practical fusion power depends on materials that survive intense neutron flux and on efficient systems to extract usable electricity.