The equation, E=mc2, is perhaps the most famous piece of information Albert Einstein introduced to the world. The equation describes the relationship between mass and energy. Prior to Einstein's work, scientists believed mass and energy were two distinct things. Einstein argued mass and energy were not necessarily separable, instead claiming the two are just different forms or qualities of the same thing. In the equation, E stands for energy, m for mass, and c for the speed of light in a vacuum. With this in mind, the equation essentially reads that energy is equal to the mass multiplied by the square of the speed of light in a vacuum.
The c2 (speed of light squared) portion of the equation represents a conversion factor. If we convert a substance's mass into pure energy, the speed of light squared establishes a dimensional constant.
Why this formula makes sense is a much bigger and more philosophical question than we can dwell upon here. Einstein's equation merely describes mass-energy relations in special relativity.