Mass–energy equivalence
E = mc²: mass is a form of energy; a little mass corresponds to an enormous amount of energy.
Einstein's 1905 result, today written as E = mc², states that the rest energy of a body equals its mass times the speed of light squared. Because c² is huge, a tiny amount of mass holds an enormous amount of energy.
It explains why the Sun shines (fusion converts mass to light), how nuclear power works, and the destructive power of nuclear weapons — though the equation itself is simply a law of nature, not a weapon.
Source: Wikipedia: Mass–energy equivalence — Read the full article on Wikipedia.