Albert Einstein says, every object at rest contains potential energy equivalent to the product of its mass times a constant, which is the speed of light squared. and in equation that is written as: E= m * c^2
"c" refers to the speed of light. Be advised the units we use in this equation are: joules (J) for energy, kilogram (kg) for mass, and we use m/s for speed of light, and when we square that we get (m/s)^2.
You know how amazing this is? Since speed of light is so huge -- 3 X 10^8 m/s, and when we square it, it is super huge. And that means the energy stored in even an extremely tiny amount of mass will be very huge.