Hi Divya, in the earlier days scientists thought that in nature energy and matter are two different entities fomed in nature. But energy ie kinetic energy is related to matter ie mass by the formula E = 1/2 * m v^2
This has given a clue to get the above relation ie E = m c^2
Here c is the velocity of light. Astonishingly, velocity of light is absolute and it will not change as the observes moves towards or away from the light source. Hence this has led to make space and time to be relative to the observer. Thus Einstein introduced the theory of relativity. In case of atomic nucleus, there are protons and neutrons. As we add the masses of these protons and neutrons we get one value. But as we determine the mass of the nucleus it seems less than the computed or expected. This is what we call mass defect. As we are able to get energy right from mass, the energy equivalent to this mass defect is known to be the binding energy of the nucleus. Thus E = mc^2 has been practically verifed bey ...more

You have to plz take up some high school physics text to get an appropriate answer for this Divya. In short, his formula tells us the nature of 'matter' and how both matter and energy are indestructible . No doubt it was one of the most fundamental discoveries of modern age.