What is the meaning of e equals mc squared?

Answer 1

E=mc2 ; E=energy m=mass c=speed of light. It simply means that energy and mass are different manifestion of the same thing. Energy can the converted into mass and vice versa. For example, a stationary proton and a moving one don't have the same mass, because the moving proton has Kinetic Energy which is equivalent to a small quantity of mass.