What physical phenomenon does E equals mc2 describe?

Answer 1

Enstein's famous equation depicts the interconvertibility of mass, m, and energy, e. In a sense, we can think of mass and energy as aspects of the same fundamental thing: mass is "congealed energy".

Because c, the velocity of light, is such a large number, it is easy to see that a small mass converts to a very large amount of energy. This is the basis of nuclear power and the fusion reactions that power stars.