E=mc^2 describes the conversion ratio between matter and energy, and as such, it has been used to aid the design of nuclear power stations and atomic weapons. It is also a useful theoretical tool that helps astrophysicists understand the reactions taking place in stars.
Einstein's most famous equation describes the amount of energy that stands to be liberated when matter is converted to energy and the amount of energy needed to synthesize matter of a given mass. This understanding has informed models of the early universe, speculations about antimatter reactions and the study of quantum gravity. The equation is also routinely used to predict the yield of fission reactions in working nuclear reactors.