According to Einstein's theory of relativity, mass and energy are related through the famous equation E = mc². This equation states that energy (E) equals mass (m) times the speed of light (c) squared, illustrating that mass can be converted into energy and vice versa. This relationship implies that a small amount of mass can produce a large amount of energy, a principle utilized in nuclear reactions. It revolutionized the understanding of mass-energy equivalence, emphasizing their interconnectedness in the fabric of the universe.