The new NIF reactor uses an array of 192 powerful laser guns aimed at a small container. Inside this container are granules of super-cooled hydrogen isotopes in the solid state. Laser beams that hit the walls of the container generate X-ray radiation that fires the sample. This process removes hydrogen atoms from the granules, incidentally heating them up to millions of degrees. As a result of the evaporation of a small amount of mass, a large amount of energy is released. A similar process occurs on the Sun.
All previous nuclear fusion reactions conducted on Earth required more energy than they eventually produced. Their inefficiency consisted in a significant consumption of resources by lasers and other sources of ignition, which was not even covered by reproducible energy. Now scientists have reached a milestone where the reaction of nuclear fusion can become self-sustaining. That is, at least, until the source of fuel has run out.