The equation E= mc^2 represents the amount of energy that a given mass contains. The equation represents mass-energy equivalence, or the idea that mass and energy are interrelated rather than being two separate things. E is the energy in joules, m represents the mass in kilograms and c is the speed of light in a vacuum, or 300,000 kilometers per second. Because the seed of light is squared, the conversion factor ends up as 90 billion.
In other words, if an object of one kilogram had its mass converted to energy, the resulting explosion would release 90 billion joules of energy. Nuclear weapons have only a small amount of their mass converted to energy, with the rest lingering as fissioned atoms and radioactive particles. The only known way to achieve a 100% conversion of mass to energy would be to bring matter into contact with a corresponding amount of antimatter.
The E in the equation refers to resting energy, or the energy that exists in the object even when it isn't in motion, being heated or otherwise irradiated with energy. Newtonian laws of motion do not apply to the object's resting energy. Conversely, photons that are traveling through space are considered to have a mass because they have kinetic energy from their movement.