Kurzgesagt has released a great video that explains Fusion Energy.  At this time Fusion Energy is not practicable as an energy source but after Elon Musk is done with kitting out the Gigafactory he has the deep enough pockets that could toy with Fusion Energy.
“Fusion power is energy generated by nuclear fusion. Fusion reactions fuse two lighter atomic nuclei to form a heavier nucleus. It is a major area of plasma physics research that attempts to harness such reactions as a source of large scale sustainable energy. Fusion reactions are how stars transmute matter into energy.
In most large scale commercial programs, heat from neutron scattering in a controlled reaction is used to operate a steam turbine that drives electric generators. Many fusion concepts are under investigation. The current leading designs are the tokamak and inertial confinement by laser. As of January 2016, these technologies were not viable, as they cannot produce more energy than is required to initiate and sustain a fusion reaction.”