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How does nuclear fusion release energy from a small amount of mass?

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laser-activated fusion — nuclear fusion Interior of the U.S. Department of Energy's National Ignition Facility (NIF), located at Lawrence Livermore National Laboratory, Livermore, California. The NIF target chamber uses a high-energy laser to heat fusion fuel to temperatures sufficient for thermonuclear ignition. The facility is used for basic science, fusion energy research, and nuclear weapons testing.

Nuclear fusion releases energy because the total mass of the resulting particles is less than the mass of the initial reactants. This process is governed by the principle that mass and energy are equivalent, as expressed by Albert Einstein’s equation $E = m c^{2}$ .12

When light elements, such as hydrogen isotopes deuterium and tritium, fuse to form heavier elements like helium, a small amount of the original rest mass is converted into energy. This occurs because the binding energy of the resulting nucleus is more efficient than that of the reactants. Specifically, in the fusion of hydrogen into helium, approximately 0.7 percent of the original rest energy is released. Because the speed of light ( $c$ ) is a very large constant, even a tiny decrease in mass results in the release of a substantial amount of energy.321