What does E equals MC² mean?

Answer 1

E=MC2

Energy equals the mass times the speed of light squared

E=energy

M=mass

C=speed of light

This is a formula for either converting mass to energy or light to equivalent matter.

The highest theoretical speed possible is the speed of light. No real matter currently known of can exceed the speed of light.

Albert Einstein created the formula to calculate the amount of energy in matter.

For example: 1 kg of water has 111 g of hydrogen atoms. So...

Energy=Mass (0.111) x the speed of light (3x10^8) squared, so...

0.111 multiplied by 3x10^8 multiplied by 3x10^8 equals

9.99x10^15 !

Hope you liked my explanation! (+correction and elaboration)
It is the mass-energy equivalent. The "e" stands for energy, the "m" stands for mass (rest- mass- basically when it is not moving) and the "c" stands for the speed of light. So to work out the energy in something you just take the speed of light (approximately 300,000 km per second in a vacuum),square it and then times it by it,the mass that it has ; this will show you that if all the mass in that object was suddenly converted to energy, how much it would be. It turns out to be a lot even for something tiny like a few molecules!
It means that any type of energy has an equivalent mass; and mass has an equivalent energy. It also provides the formula to calculate one, on the basis of the other (the proportionality constant).