If a body gives off the energy L in the form of radiation, its mass diminishes by L/V2.The V in this case is the 1920s-era standard variable for the speed of light (which Einstein argued was constant). Thus, if you wrote out the mass-energy equivalence equation as Einstein originally described it, you'd get m = L/V2.
The upshot of Einstein's mass-energy equivalence and the relativity it helps describe is that all matter can be converted into a predictable amount of energy -- a large predictable amount of energy. Fortunately, only in very rare circumstances can matter be efficiently and explicitly converted entirely into its equivalent energy. We don't unleash all of our food energy when we digest it, for example, because we're unlocking its chemical energy, not its nuclear energy. That's a very good thing, as E = mc2 would make your average slice of cheesecake exponentially more fattening (and destructive).
According to Einstein's famous equation, how many food calories are there in a single gram of mass?