His famous equation, E =mc 2 , did not appear with the paper, but came in a brief supplement that followed a few months later. As you will recall from school days, E in the equation stands for energy, m for mass, and c 2 for the speed of light squared.
In simplest terms, what the equation says is that mass and energy have an equivalence. They are two forms of the same thing: energy is liberated matter; matter is energy waiting to happen. Since c 2 (the speed of light times itself) is a truly enormous number, what the equation is saying is that there is a huge amount—a really huge amount—of energy bound up in every material thing.