"E=mc2" is one of Albert Einstein's most famous equations, which relates the energy and mass of an object. An important thing to note is that since mass is multiplied by the speed of light squared, even small amounts of mass can create large amounts of energy.
The realization that energy and mass are similar quantities has been key to many physics breakthroughs of the 20th century. The ability to unleash large amounts of energy by breaking small amounts of mass was the concept behind atomic power. The mass-energy equivalence implied by E=mc2 is a result based on Einstein's broader theories of special and general relativity.