e=mc2, also called the "mass-energy equivalence", is one of the things that einstein is most famous for. it is a famous equation in physics and math that shows what happens when mass changes to energy or energy changes to mass. the "e" in the equation stands for energy. energy is a number which you give to objects depending on how much they can change other things. for instance, a brick hanging over an egg can put enough energy onto the egg to break it, but a feather can not.