The c in Einstein's famous equation stands for the speed of light. Light travels constantly at a speed of 186,000 miles per second, meaning that c equals 186,000 miles per second.
This equation comes from Einstein's theory of special relativity. The equation shows that the mass (m) of an object is determined by its kinetic energy (E) divided by the speed of light (c) squared. Einstein created this equation to show that mass and energy are interchangeable. With this equation, he was able to argue that mass and energy are actually the same thing, and to convert one to the other, you simply need to factor in the speed of light.