The famous equation by Einstein is not so easy to explain. Basically, it says that mass and energy are equivalent.
For example, when the nucleus of Uranium-235 decays(breaks down) into two daughter nuclei, the sum of masses of the daughter nuclei is less than that of the original U-235 nucleus. Instead, we get energy which is equal to the “missing” mass times speed of light squared.
For more technical explanation, see https://www.youtube.com/watch?v=Xo232kyTsO0
For more equations in Physics, see Famous Equations in Physics.