Einstein's formula tells us the amount of energy this mass would be equivalent to, if it were all suddenly turned into energy. Although we don't normally think about it, everything, including the air we breathe has a mass (weight). "M" in the equation stands for the Mass. C2 stands for the speed of light, squared. (or you can think about it as "speed of light X speed of light) (which is always 900,000,000) It says that to find the energy, you multiply the mass by the square of the speed of light. The answer will be measured in Joules, because energy is measured in joules.
The simplest explanation: For any given quantity of matter, there is an equivalent quantity of energy. Putting energy into something makes it heavier. Taking energy out of something makes it lighter.